Has many fetch collection check

A get request that receives no data returns an instance. However, the fetch method has_many_association implicitly assumes it will receive a collection. When it instead receives an instance https://github.com/remiprev/her/blob/master/lib/her/model/associations/has_many_association.rb#L88 
attempts to iterate over the instance and raises an exception. 

This patch ensures that we always operate on a collection and allows delegated methods to work even when the response from the get request is blank. e.g. fixes https://github.com/remiprev/her/issues/187
wouldn't it be better if this'd be done when calling `super`?