If auth already in requests, don't clobber it

[dpocock]

As an alternative, maybe it should throw if auth is present in both requests and the explicit username/password at the same time.

[coveralls]

Coverage remained the same at 98.678% when pulling 336e4109a1895f19fcad4bd6891f8377c42a37f0 on dpocock:dpocock/auth-in-requests into 02410b36801e74c86b555df33cbc5284c1112940 on maxtepkeev:master.

[maxtepkeev]

I don't think we should fix this or check for such situations, because:

1. If someone wants to use auth key for providing username and password he can do so
2. If someone wants to use username and password kwargs in Python-Redmine he can also do so
3. If someone tries to use both methods simultaneously for some weird reason especially providing different username and password in kwargs and auth key for requests, well, it's really his code's problem and this is where a fix should be made