Extract #fetch_and_decode method to encapsulate logic for retrieving a resource and decoding it

I'm working on an alternative `ActiveResource::Format` to decode [LDP](http://w3.org/2012/ldp/wiki/Main_Page) RDF responses. The standard defines some HTTP headers that may contain additional information that may present themselves as attributes in the model. 

In this patch, I've extracted `#fetch_and_decode` to encapsulate the logic I'll need to override to send the response headers to the format decoder.