fix(ngSrc, ngSrcset, ngHref): only interpolate if all expressions are defined

BREAKING CHANGE

If `bar` is `undefined`, before `<img src="foo/{{bar}}.jpg">` yields
`<img src="foo/.jpg">`. With this change, the binding will not set `src`.

If you want the old behavior, you can do this: `<img src="foo/{{bar || ''}}.jpg">`.

The same applies for `srcset` and `href`.

Closes angular#6984

Author: btford

src/ng/directive/booleanAttrs.js

@@ -365,7 +365,7 @@ forEach(BOOLEAN_ATTR, function(propName, attrName) {
 // ng-src, ng-srcset, ng-href are interpolated
 forEach(['src', 'srcset', 'href'], function(attrName) {
   var normalized = directiveNormalize('ng-' + attrName);
-  ngAttributeAliasDirectives[normalized] = function() {
+  ngAttributeAliasDirectives[normalized] = ['$interpolate', '$sce', function($interpolate, $sce) {
     return {
       priority: 99, // it needs to run after the attributes are interpolated
       link: function(scope, element, attr) {
@@ -379,11 +379,21 @@ forEach(['src', 'srcset', 'href'], function(attrName) {
           propName = null;
         }
 
-        attr.$observe(normalized, function(value) {
-          if (!value)
-             return;
+        var parsed = $interpolate(element.attr(attr.$attr[normalized]));
+
+        scope.$watchGroup(parsed.expressions, function (values) {
+          var merged = '';
+          for (var i = values.length - 1; i >= 0; i -= 1) {
+            if (isDefined(values[i])) {
+              merged += parsed.separators[i] + values[i];
+            } else {
+              return;
+            }
+          }
 
-          attr.$set(name, value);
+          merged += parsed.separators[values.length];
+
+          attr.$set(name, merged);
 
           // on IE, if "ng:src" directive declaration is used and "src" attribute doesn't exist
           // then calling element.setAttribute('src', 'foo') doesn't do anything, so we need
@@ -393,5 +403,5 @@ forEach(['src', 'srcset', 'href'], function(attrName) {
         });
       }
     };
-  };
+  }];
 });

test/ng/directive/booleanAttrsSpec.js

@@ -204,7 +204,7 @@ describe('ngSrcset', function() {
     var element = $compile('<div ng-srcset="some/{{id}} 2x"></div>')($rootScope);
 
     $rootScope.$digest();
-    expect(element.attr('srcset')).toEqual('some/ 2x');
+    expect(element.attr('srcset')).toEqual();
 
     $rootScope.$apply(function() {
       $rootScope.id = 1;
@@ -227,7 +227,7 @@ describe('ngHref', function() {
   it('should interpolate the expression and bind to href', inject(function($compile, $rootScope) {
     element = $compile('<div ng-href="some/{{id}}"></div>')($rootScope);
     $rootScope.$digest();
-    expect(element.attr('href')).toEqual('some/');
+    expect(element.attr('href')).toEqual();
 
     $rootScope.$apply(function() {
       $rootScope.id = 1;
@@ -258,7 +258,7 @@ describe('ngHref', function() {
         element = $compile('<svg><a ng-href="some/{{id}}"></a></svg>')($rootScope);
         var child = element.children('a');
         $rootScope.$digest();
-        expect(child.attr('xlink:href')).toEqual('some/');
+        expect(child.attr('xlink:href')).toEqual();
 
         $rootScope.$apply(function() {
           $rootScope.id = 1;

test/ng/directive/ngSrcsetSpec.js

@@ -11,6 +11,6 @@ describe('ngSrcset', function() {
     $rootScope.image = {};
     element = $compile('<img ng-srcset="{{image.url}} 2x">')($rootScope);
     $rootScope.$digest();
-    expect(element.attr('srcset')).toEqual(' 2x');
+    expect(element.attr('srcset')).toEqual();
   }));
 });