Users users model - postgresql (case sensitive DDL)

#### Steps to reproduce the issue
On administration side
go to  User -> User Manager  and click to sort for Last Visit Date or Registration Date column

#### Expected result
The user list is sorted by registration or visit date

#### Actual result
but it throws a Database Error
 query failed (error # %s): %s SQL=SELECT a.* FROM "#_users" AS a ORDER BY a.registerDate ASC LIMIT 20

#### System information
Joomla! 3.4.0-alpha
Postgres 9.3.5
PHP 5.5.9

#### Additional comments
Postgres is a little bit different than mysql on mixed case columns name , so this is is only a temporary fix
should be better if all joomla fields became lowercase,  the user table is the only table that have mixed case field