get_api_call_arg had incorrectly returned None if a kwarg was passed
to the mocked function with a False-y value (e.g., [] or {})
get_api_call_json had only considered data param, ignoring json param
requests added a while back
SendGrid: update to v3 send API
**SendGrid:** **[possibly-breaking]** Update SendGrid backend to newer Web API v3. This should be a transparent change for most projects. Exceptions: if you use SendGrid username/password auth, esp_extra with "x-smtpapi", or multiple Reply-To addresses, please review the [porting notes](http://anymail.readthedocs.io/en/latest/esps/sendgrid/#sendgrid-v3-upgrade).
Closes#28