Fix json bodies

Fixes #49 

Seems that params is just the wrong attribute to use ([docs](http://docs.pylonsproject.org/docs/pyramid/en/1.5-branch/api/request.html#pyramid.request.Request.params))! 

Added a new test to cover this use case and renamed the existing acceptance test modules to better reflect their contents.
I believe this bug surfaced when I converted acceptance tests from using webtest to use requests (and making real http requests to a running app). I believe that suggested that webtest is potentially masking some errors (possible in rare cases).

Does this test fail with the old code?