Unit test fixes

I ran [pitest](http://pitest.org) and found two things:
1. An order dependency in the WebsocketControllerTest due to the static fields
2. Some assertThat lines that were not actually asserting anything

Using the pitest results, I added some more assertions and tests - mostly for error cases - in a [branch](https://github.com/mdietz198/TextSecure-Server/tree/add-pitest). If that help is appreciated, I can keep adding some more coverage and make another pull request. If not, no worries. If you're more interested in contributions to the client, I'd be happy to give that a shot.