The `response` attribute is not set when response type is `json`

[JakeDluhy]

When making my actual API requests to my backend server (Rails), the xhr has the response key, which is the parsed data. However the FakeXMLHttpRequest (I'm using pretender) does not have it. I originally was using the responseText property and parsing it myself, but the responses from my server don't have it.

My guess is this might also have something to do with my client xhr library (I'm using rxjs5).

But I think it makes sense to have the parsed response attribute (especially since I need it ;) ).

Hoping you'll accept a PR, gonna try and get it out tonight

[JakeDluhy]

I just updated my PR on top of master. Any chance I could get the changes merged in?

[robclancy]

This is annoying... 2017.