PHP "Notice" and unclear parsing of headers/Status

Hi Nil,

I recently tried to integrate your restclient-component into a project I'm currently working on and regularly got a

    PHP Notice:  Undefined index: headers in [...]/restclient-component/src/Sonrisa/Component/RestfulClient/AbstractClient.php on line 27

I decided to check the code and ended up in creating this pull request with which I provide a slightly modified (non bc breaking) `AbstractClient.php` code for situations where the response type actually is `application/json` (not `application/x-javascript` like returned by duckduckgo.com used in the tests). I also added an additional test for asserting GET requests with Content-Type `application/json`.

I added an inline comment to clarify my interpretation and intention. I'm still not sure what your original intention might have been...

Cheers, 
atufkas