Forcing content type on the sendErrorResource the same way the sendResou...

Hi,
I've run into the following problem - I am forcing the default contentType of API response by providing an explicit $contentType value to ResourcePresenter::sendResource($contentType).

However ResourcePresenter::sendErrorResource() method works differently and does not provide a way to force the content type and always relies on the Accept http headers only. This seems a bit unfortunate, thus I have prepared this pull request which unifies the way both methods work.

Btw, great library. :)