Clarify handling of JSON request bodies
Clarify handling of JSON request bodies

Two things to simplify handling of JSON request bodies:

1. Add a utility, `has_json_body()`, that returns true if the
   request has a JSON body.
2. Improve comments for `get_json_body()` to clarify that
   if the request has no body or a non-JSON body, the function
   will return an empty map.

CMPT276-2016spring/phaser,CMPT276-2016spring/phaser,CMPT276-2016spring/phaser