Testing Docs

The method signatures for `RapidTest.send` and `RapidTest.receive` in the testings docs are backwards/incorrect:

- https://rapidsms.readthedocs.org/en/v0.11.1/topics/testing.html#RapidTest
- https://github.com/rapidsms/rapidsms/blob/v0.11.1/rapidsms/tests/harness/router.py#L33

`receive` has the `fields` argument and the `connection` should be singular.