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

@@ -379,6 +379,7 @@ forEach(['src', 'srcset', 'href'], function(attrName) {
           propName = null;
         }
 
+        attr.$setAllOrNothing(normalized);
         attr.$observe(normalized, function(value) {
           if (!value)
              return;

test/ng/directive/booleanAttrsSpec.js

@@ -202,9 +202,8 @@ describe('ngSrc', function() {
 describe('ngSrcset', function() {
   it('should interpolate the expression and bind to srcset', inject(function($compile, $rootScope) {
     var element = $compile('<div ng-srcset="some/{{id}} 2x"></div>')($rootScope);
-
     $rootScope.$digest();
-    expect(element.attr('srcset')).toEqual('some/ 2x');
+    expect(element.attr('srcset')).toBeUndefined();
 
     $rootScope.$apply(function() {
       $rootScope.id = 1;
@@ -227,7 +226,7 @@ describe('ngHref', function() {
   it('should interpolate the expression and bind to href', inject(function($compile, $rootScope) {
     element = $compile('<div ng-href="some/{{id}}"></div>')($rootScope);
     $rootScope.$digest();
-    expect(element.attr('href')).toEqual('some/');
+    expect(element.attr('href')).toBeUndefined();;
 
     $rootScope.$apply(function() {
       $rootScope.id = 1;
@@ -258,7 +257,7 @@ 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')).toBeUndefined();
 
         $rootScope.$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')).toBeUndefined();
   }));
 });