Fix displaying RawQuerySet in sql_explain view

[todorvelichkov]

This problem may be a regression from #309, but I'm not 100% sure.

Trying to do sql_explain on raw query with dictionary for params 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.

[matthiask]

Thank you!