fix(accordion): Allow accordion heading directives as attributes.

Closes #540

src/accordion/accordion.js

@@ -104,7 +104,7 @@ angular.module('ui.bootstrap.accordion', ['ui.bootstrap.collapse'])
 // </accordion-group>
 .directive('accordionHeading', function() {
   return {
-    restrict: 'E',
+    restrict: 'EA',
     transclude: true,   // Grab the contents to be used as the heading
     template: '',       // In effect remove this element!
     replace: true,

src/accordion/test/accordionSpec.js

@@ -284,6 +284,28 @@ describe('accordion', function () {
 
     });
 
+    describe('accordion-heading attribute', function() {
+      beforeEach(function() {
+        var tpl = 
+          '<accordion ng-init="a = [1,2,3]">' +
+            '<accordion-group heading="I get overridden">' +
+              '<div accordion-heading>Heading Element <span ng-repeat="x in a">{{x}}</span> </div>' +
+              'Body' +
+            '</accordion-group>' +
+          '</accordion>';
+        element = $compile(tpl)(scope);
+        scope.$digest();
+        groups = element.find('.accordion-group');
+      });
+      it('transcludes the <accordion-heading> content into the heading link', function() {
+        expect(findGroupLink(0).text()).toBe('Heading Element 123 ');
+      });
+      it('attaches the same scope to the transcluded heading and body', function() {
+        expect(findGroupLink(0).find('span').scope().$id).toBe(findGroupBody(0).find('span').scope().$id);
+      });
+
+    });
+
     describe('accordion-heading, with repeating accordion-groups', function() {
       it('should clone the accordion-heading for each group', function() {
         element = $compile('<accordion><accordion-group ng-repeat="x in [1,2,3]"><accordion-heading>{{x}}</accordion-heading></accordion-group></accordion>')(scope);
@@ -295,5 +317,19 @@ describe('accordion', function () {
         expect(findGroupLink(2).text()).toBe('3');
       });
     });
+
+
+    describe('accordion-heading attribute, with repeating accordion-groups', function() {
+      it('should clone the accordion-heading for each group', function() {
+        element = $compile('<accordion><accordion-group ng-repeat="x in [1,2,3]"><div accordion-heading>{{x}}</div></accordion-group></accordion>')(scope);
+        scope.$digest();
+        groups = element.find('.accordion-group');
+        expect(groups.length).toBe(3);
+        expect(findGroupLink(0).text()).toBe('1');
+        expect(findGroupLink(1).text()).toBe('2');
+        expect(findGroupLink(2).text()).toBe('3');
+      });
+    });
+
   });
 });