Submit JSON HTTP callbacks as the sole body.

Instead of storing JSON data in a "body" POST field the payload should be set as
the entire body itself.

Thanks to @DylanGriffith for bringing this up.

Closes #1 #2
Corrected setting of headers for HTTP callbacks.

HTTPClient confuses the heck out of me sometimes. Apparently when using a POST
request the headers are just the 3rd argument instead of having to be placed in
a {:header => ...} Hash.