Selenium appears to be sending a HEAD request instead of a GET which explains why you are getting 404 when you restrict the action to HttpGet.
To confirm, place a breakpoint in Logon() and examine the Method property of the Request.
This appears to be an incompatibility between selenium and MVC.
CassiniDev appears to behave as expected.
Please confirm and report back what steps you take to configure Selenium to send a GET instead of HEAD as this will likely be of interest to others as well.
Also, I would suggest removing the .StopServer() in the teardown. I mistakenly left that in for the .lib fixture examples.
It is only relevant to out-of-proc cassinidev.exe.
This will eliminate spurious exceptions that you may have to pass through at the end of a debugging run.