Return value of delete_user($username)

[Yoshimasa Iwase]

According to function comment as below, delete_user($username) should return boolean value.

auth/classes/auth/login/ormauth.php

Line 481 in 61e3c14

 	 * @return  bool 

However, it seems that delete user($username) returns "Model this instance as a new object without primary key(s)" shown as below:

https://github.com/fuel/orm/blob/e8457ba9296078310cdea47002e5c7b7cb083da2/classes/model.php#L1456

I think that delete_user($username) should return boolean value as documented.

[Harro Verton]

Duplicate of #86