Fix bad update_username test

The update_username test was checking for "success: true" json response
from the server, but the server was returning an empty json body. I
updated the server json response in [PR discourse/discourse #3035](https://github.com/discourse/discourse/pull/3035) to return the updated username.

The following changes were made:

- check that the updated username is returned
- update fixture to return the updated username
- remove redundant api_key and api_username