Pass falsy data to ClientRequest.update_body_from_data handler

While logic of processing only "true" values to form request body is ok
for plain raw requests, it causes troubles when you inherit from
ClientRequest to produce requests for some specific content type.
For instance, it's ok to pass "falsy" `0`, `False`, `{}` data if you're
implementing JsonClientRequest class that automatically encodes data
inside update_body_from_data method.
Merge pull request #143 from kxepal/handle-falsy-data-on-request

Pass falsy data to ClientRequest.update_body_from_data handler