Add check for body type

Following from: https://github.com/AmpersandJS/ampersand-model/issues/12 we were getting the same issue with collections since they were both using similar fetch() functions.

This checks if the returned body is a json object or not. On fetches it should return some data, or null. In the case of null fetch() should return some form of error. (internet disruptions, unauthorized access, etc).

Thoughts?

cc @funkytek