Fix RPC requests with no POST data

The parameter to get_request_data is the default value if there is no body, not
the request. As it was, it would end up with effectively handler(**request) in
the self.execute call, a TypeError as kwargs expansion only works on mappings.
Add RPC tests and improve relevant error handling.
Merge branch 'patch-1' of https://github.com/chris-morgan/django-nap into chris-morgan-patch-1

Conflicts:
	runtests.py
Merge branch 'chris-morgan-patch-1'