filter: Fix that leading and trailing whitespaces for columns and expressions let filters fail

Before, filtering for "host = localhost" issued "got invalid column host".

library/Icinga/Data/Filter/Filter.php

@@ -131,6 +131,8 @@ abstract class Filter
 
     public static function expression($col, $op, $expression)
     {
+        $col = trim($col);
+        $expression = trim($expression);
         switch ($op) {
             case '=': return new FilterMatch($col, $op, $expression);
             case '<': return new FilterLessThan($col, $op, $expression);