Fixing handling of http reponse errors

This commit fix the error of rising a wrong HTTPParseException when
trying to parse the body for non 200 response status code

Now we first doublecheck the status codes for the response, then we
check if content-type header is json and rise the parse execption if not

We also ensure that if status code is not 200 we don't pollute the
attributes for the current object.
Merge pull request #38 from whichdigital/fixing-response-parsing

Fixing handling of http reponse errors
Merge branch 'develop' of github.com:whichdigital/active-rest-client into develop