Login capitalization reconciliation

Our login procedure now uses the account name that the server resolves from the one that the user entered, rather than the one that the user entered.

The existing tests exposed an over-the-edge case that the existing logic failed to cover (though the server never actually does the covered behavior, hence "over-the-edge case" (-: ).

I've added a test that establishes the login procedure's respect for the server's resolution of the user name.

(The [server's login API documentation](https://spideroak.com/faq/questions/37/how_do_i_use_the_spideroak_web_api/) is missing a bunch of essential details about the login protocol.)