Null request body sent as 'null' string

[grimsa]

We observed this after upgrading apisauce from 1.1.2 to 2.1.5

We had code similar to apisauceInstance.post('some-url', null, configObject) which then resulted in a null (as a four-letter string) being sent as a request body.

Changing to apisauceInstance.post('some-url', undefined, configObject) reverted to the expected behavior (no request body sent).

This seems to be similar to #9