Bug 874348 - Fix some issues with autoconfiguration that made it impossible to set up an account at 163.com

r? @asutherland

Do you think these changes are sane? I tested it via the examples/email.html page, but it should work fine in gaia too.

We could also make our logic in the open function simpler, and never add the Microsoft-ActiveSync-Server path, but that would require writing some migration code for gaia email account settings and updating all the autoconfig files for ActiveSync. Let me know if you think that's a better way to do things.