Authenticating should not override chosen location

The authentication process now includes support for pluggable authentication schemes, and these authentication schemes may take responsibility over setting the user's session location from the login form.

However, the current implementation of the UserContext#authenticate method always sets the sessionLocation to the user's default location (even if the default location is null and the current session location is non-null), due to the way in which it is implemented and when it is invoked.

This should be fixed such that the default session location is only set if the session location is not already non-null.

ibacher FYI, as it looks like you worked on this code to address a similar issue in TRUNK-6141 last year.