WebSession: loginUser() fixes
- Fixes return statements in loginUser() API function after modification introduced in bb52827c8badd881167547c22b2e189288f25719. (closes #1159)
- Correctly handles the merging of userEXT table when two accounts are merged.
- Fixes test_robot_login_method_merging_accounts regression test which assumed demo site ZRobot login method was not enforcing external nicknames.