When finding user by email, should return only single user

1) finding user by ```email``` will return a unique user, (as there is a uniqueness validation in place).
2) Also this would remove the requirement to do a ```.first``` at any place where the user is being looked up by email.

A question:
finding user by ```username```  here checks for ```users.size``` , is this the intended behavior ?, as there seems to be a validation too, checking for uniqueness of ```username``` according to which, should  not ``` find_by_username```  return a single user instead of an array of users? 

Feedback? 
@eviltrout @SamSaffron