Issue #51: Update extensionMap()

If user override existing extension, angular properties ($) will be preserved.

This piece of logic could be refactored into separate method:
Something like we have extend(), addMissingProperties() - I can't find a name
for this method...

Closes #51

src/Angular.js

@@ -281,11 +281,18 @@ function inherit(parent, extra) {
 function noop() {}
 function identity($) {return $;}
 function valueFn(value) {return function(){ return value; };}
+
 function extensionMap(angular, name, transform) {
   var extPoint;
   return angular[name] || (extPoint = angular[name] = function (name, fn, prop){
     name = (transform || identity)(name);
     if (isDefined(fn)) {
+      if (isDefined(extPoint[name])) {
+        foreach(extPoint[name], function(property, key) {
+          if (key.charAt(0) == '$' && isUndefined(fn[key]))
+            fn[key] = property;
+        });
+      }
       extPoint[name] = extend(fn, prop || {});
     }
     return extPoint[name];

test/AngularSpec.js

@@ -280,3 +280,23 @@ describe('angularJsConfig', function() {
                                           ie_compat_id: 'ng-ie-compat'});
   });
 });
+
+describe('extensionMap', function() {
+  it('should preserve $ properties on override', function() {
+    var extension = extensionMap({}, 'fake');
+    extension('first', {$one: true, $two: true});
+    var result = extension('first', {$one: false, $three: true});
+
+    expect(result.$one).toBeFalsy();
+    expect(result.$two).toBeTruthy();
+    expect(result.$three).toBeTruthy();
+  });
+  
+  it('should not preserve non-angular properties', function() {
+    var extension = extensionMap({}, 'fake');
+    extension('first', {two: true});
+    var result = extension('first', {$one: false, $three: true});
+
+    expect(result.two).not.toBeDefined();
+  });
+});