Clean up xbee base unit tests

[hansmosh]

1. We shouldn't need to be calling into private methods like _write: https://github.com/nioinnovation/python-xbee/blob/master/xbee/tests/test_base.py#L28
2. Are these _write tests even necessary? ieee tests use send which calls into write so it seems like _write is just being tested unnecessarily here.