Fix displaying RawQuerySet in sql_explain view

Trying to do sql_explain on raw query with dictionary causes a crash.

```
ERROR Internal Server Error: /__debug__/sql_explain/
Traceback (most recent call last):
  File "../site-packages/django/core/handlers/exception.py", line 41, in inner
    response = get_response(request)
  File "../site-packages/django/core/handlers/base.py", line 187, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "../site-packages/django/core/handlers/base.py", line 185, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "../site-packages/django/views/decorators/csrf.py", line 58, in wrapped_view
    return view_func(*args, **kwargs)
  File "../site-packages/debug_toolbar/decorators.py", line 15, in inner
    return view(request, *args, **kwargs)
  File "../site-packages/debug_toolbar/panels/sql/views.py", line 61, in sql_explain
    cursor.execute("EXPLAIN %s" % (sql,), params)
  File "../site-packages/debug_toolbar/panels/sql/tracking.py", line 188, in execute
    return self._record(self.cursor.execute, sql, params)
  File "../site-packages/debug_toolbar/panels/sql/tracking.py", line 121, in _record
    return method(sql, params)
  File "../site-packages/django/db/backends/utils.py", line 79, in execute
    return super(CursorDebugWrapper, self).execute(sql, params)
  File "../site-packages/django/db/backends/utils.py", line 64, in execute
    return self.cursor.execute(sql, params)
  File "../site-packages/django/db/backends/mysql/base.py", line 101, in execute
    return self.cursor.execute(query, args)
  File "../site-packages/MySQLdb/cursors.py", line 159, in execute
    query = query % db.literal(args)
TypeError: format requires a mapping
```

The reason is because `params` is not dictionary, but list.

The core of the issue is caused by `panels/sql/traking.py`
```
    _params = json.dumps([self._decode(p) for p in params])
```
This forces only dictionary keys to be send over sql_explain view.

My change is going to preserve the initial params structure.
```
    _params = json.dumps(self._decode(params))
```

Now, because dictionary params are not supported by the SQLite backend.
My test is going to be skipped for this vendor.

Author: todorvelichkov

debug_toolbar/panels/sql/tracking.py

@@ -124,7 +124,7 @@ def _record(self, method, sql, params):
                 stacktrace = []
             _params = ''
             try:
-                _params = json.dumps([self._decode(p) for p in params])
+                _params = json.dumps(self._decode(params))
             except TypeError:
                 pass  # object not JSON serializable
 

tests/panels/test_sql.py

@@ -122,6 +122,55 @@ def test_param_conversion(self):
             '["2017-12-22 16:07:01"]'
         ))
 
+    @unittest.skipUnless(connection.vendor != 'sqlite',
+                         'Test invalid for SQLite')
+    def test_raw_query_param_conversion(self):
+        self.assertEqual(len(self.panel._queries), 0)
+
+        list(User.objects.raw(
+            " ".join([
+                "SELECT *",
+                "FROM auth_user",
+                "WHERE first_name = %s",
+                "AND is_staff = %s",
+                "AND is_superuser = %s",
+                "AND date_joined = %s",
+            ]),
+            params=['Foo', True, False, datetime.datetime(2017, 12, 22, 16, 7, 1)],
+        ))
+
+        list(User.objects.raw(
+            " ".join([
+                "SELECT *",
+                "FROM auth_user",
+                "WHERE first_name = %(first_name)s",
+                "AND is_staff = %(is_staff)s",
+                "AND is_superuser = %(is_superuser)s",
+                "AND date_joined = %(date_joined)s"
+            ]),
+            params={
+                'first_name': 'Foo',
+                'is_staff': True,
+                'is_superuser': False,
+                'date_joined': datetime.datetime(2017, 12, 22, 16, 7, 1)},
+        ))
+
+        self.panel.process_response(self.request, self.response)
+        self.panel.generate_stats(self.request, self.response)
+
+        # ensure query was logged
+        self.assertEqual(len(self.panel._queries), 2)
+
+        self.assertEqual(tuple([q[1]['params'] for q in self.panel._queries]), (
+            '["Foo", true, false, "2017-12-22 16:07:01"]',
+            " ".join([
+                '{"first_name": "Foo",',
+                '"is_staff": true,',
+                '"is_superuser": false,',
+                '"date_joined": "2017-12-22 16:07:01"}'
+            ])
+        ))
+
     def test_insert_content(self):
         """
         Test that the panel only inserts content after generate_stats and

tests/settings.py

@@ -91,6 +91,17 @@
             'NAME': 'debug-toolbar',
         }
     }
+elif os.environ.get('DJANGO_DATABASE_ENGINE') == 'mysql':
+    # % mysql
+    # mysql> CREATE USER 'debug_toolbar'@'localhost' IDENTIFIED BY '';
+    # mysql> GRANT ALL PRIVILEGES ON debug_toolbar.* TO 'test_debug_toolbar'@'localhost';
+    DATABASES = {
+        'default': {
+            'ENGINE': 'django.db.backends.mysql',
+            'NAME': 'debug_toolbar',
+            'USER': 'debug_toolbar',
+        }
+    }
 else:
     DATABASES = {
         'default': {