revert: fix(ngHref): remove attribute when empty value instead of ignoring

This reverts commit 948c86c602.

This commit caused tests to fail in IE8 due to a TypeError '0.childNodes is
null or not an object. The issue should be investigated and fixed. Issue #8340
has been opened to investigate.

See this job for failures: https://travis-ci.org/angular/angular.js/jobs/30792508

src/ng/directive/booleanAttrs.js

@@ -380,12 +380,8 @@ forEach(['src', 'srcset', 'href'], function(attrName) {
         }
 
         attr.$observe(normalized, function(value) {
-          if (!value) {
-            if (attrName === 'href') {
-              attr.$set(name, null);
-            }
-            return;
-          }
+          if (!value)
+             return;
 
           attr.$set(name, value);
 

test/ng/directive/booleanAttrsSpec.js

@@ -252,23 +252,6 @@ describe('ngHref', function() {
     expect(element.attr('href')).toEqual('http://server');
   }));
 
-  it('should not set the href if ng-href is empty', inject(function($rootScope, $compile) {
-    $rootScope.url = null;
-    element = $compile('<a ng-href="{{url}}">')($rootScope);
-    $rootScope.$digest();
-    expect(element.attr('href')).toEqual(undefined);
-  }));
-
-  it('should remove the href if ng-href changes to empty', inject(function($rootScope, $compile) {
-    $rootScope.url = 'http://www.google.com/';
-    element = $compile('<a ng-href="{{url}}">')($rootScope);
-    $rootScope.$digest();
-
-    $rootScope.url = null;
-    $rootScope.$digest();
-    expect(element.attr('href')).toEqual(undefined);
-  }));
-
   if (isDefined(window.SVGElement)) {
     describe('SVGAElement', function() {
       it('should interpolate the expression and bind to xlink:href', inject(function($compile, $rootScope) {