UpdateAccount in FlexMembershipProvider can create multiple users with same username

[rickardliljeberg]

FlexMembershipProvider.UpdateAccount does not check if the username will collide before happily changing it.

Making it possible to make two users have the same username, which then makes the system grind to a halt.

[OdeToCode]

Opps - will try to clean this up.

[rickardliljeberg]

Do you think we can move forward here, would you prefer if I did it and made a pull request?

[OdeToCode]

Yes, sorry. I forgot about this, I probably won't be able to get to it for a week...

[rickardliljeberg]

I created a pull request that solves this, only tested with EF

#39