Add a test for nested resources from a `addRestangularMethod` result

This test identifies an issue I am currently having with `addRestangularMethod`.

Given the following code.

```js
var Accounts = Restangular.withConfig(function(RestangularConfigurer) {
  RestangularConfigurer.addElementTransformer('users', true, function(worker) {
    worker.addRestangularMethod('doSomething', 'get', 'do-something');
    return worker;
  });
}).service('accounts');

Accounts.doSomething().then(function(doSomething) {
  doSomething.one('foo').get();
});
```

I would expect `doSomething.one('foo').get()` to fetch `accounts/do-something/foo`,
and not `accounts/foo`.
Fix #902
Add tests for addRestangularMethod(..., post/put/delete, ...);
Fix `addRestangularMethodFunction` to handle post/put/delete properly