Extend function fix

Extend function of Angular blindly overwrites destination values even if source value is undefined. This fix allows "default" values to remain intact if new value is undefined.

Author: stsvilik

src/Angular.js

@@ -233,7 +233,9 @@ function extend(dst) {
   forEach(arguments, function(obj){
     if (obj !== dst) {
       forEach(obj, function(value, key){
-        dst[key] = value;
+        if(value !== undefined) {
+          dst[key] = value;
+        }
       });
     }
   });