fix($resource): Apply request interceptors

Request Type: bug

How to reproduce: Apply request/requestError interceptors on a $resource. It will never get called

Component(s): ngResource

Impact: small

Complexity: small

This issue is related to: 

**Detailed Description:**

ngResource previously ignored 'request' and 'requestError' interceptors.
I've added the ability to use both with $resource, althought
I'm not sure when (or if) requestError is supposed to be fired.
This is also the reason why I didn't include any tests.
I don't know what triggers a requestError.

**Other Comments:**