Enable non-object JSON bodies

Updated JSON requests so they can contain bodies that are not objects
I wouldn't mind if we have `t.deepEqual(value, body)` here just to make sure (and clear) we're parsing back the value correctly. Currently that's not possible because `createPostValidator` is returning plain text, and in the case of this test it's checking only for the presence of the `content-type` header and if the value was stringified correctly.