fix($resource): return result from prototype methods


1. ```Resource[name]``` returns ```value``` that contains ```$then``` and other useful things. [#L476](https://github.com/angular/angular.js/blob/master/src/ngResource/resource.js#L476)
2. ```Resource.prototype['$' + name]``` invokes ```Resource[name]``` at the end but returns nothing. [#L502](https://github.com/angular/angular.js/blob/master/src/ngResource/resource.js#L502)

__Possible fix:__  add ```return``` state to the end of the ```Resource.prototype['$' + name]``` definition.