Create integration test for generated request using test token.

[azmb]

In order to avoid bugs like: https://github.com/messagebird/go-rest-api/pull/23 we need integration test that will talk to Messagebird API. We need to compare response data to requested data.

[epels]

Although it does not actually hit the API, this PR adds support for asserting a certain endpoint was called (method + path). Additionally, it allows for inspecting request bodies generated by the client.