Your association is mis-named, and either rails, shoulda-matchers or both are having a hard time guessing the class name from it.
You have two choices. First, and preferable is to rename the association to be conventional:
has_one :user_reset_password
This will allow rails to correctly guess the classname UserResetPassword
Second, you could simply remove the guesswork and tell rails what the classname is. This is only preferable if you can't or very much do not want to change the association name.
has_one :userresetpassword, :class_name => "UserResetPassword"