added some basic tests of the core request() method using dropbox/responses

This PR is for comment only at this time.

The unit tests that I've added are more systematic in the testing of the request creation by Twython. Each unit test is divided into setup, execute, and verify phases (separated by a newline). The tests are named to describe what they test and should be read by removing the word test:

test_request_should_encode_boolean_as_lowercase_string => request should encode a boolean as a lowercase string