Fix Bug in Database Module which causes WHERE compiling to fail

Currently the DB Module uses the $db->quote_identifier(reset($column)) in the query builder when compiling the where columns. However this function should only be used on the column alias and not the column name as it can not handle quoting mysql functions and other unique sql statements. 

$db->quote_column(reset($column)) works as expected. The variable that caused the issue is

array('CONCAT("column1", "column2")', 'alias')