Don't trim filter expressions

Leading and trailing whitespaces may be significant for comparison.

refs #8777

library/Icinga/Data/Filter/FilterExpression.php

@@ -12,7 +12,6 @@ class FilterExpression extends Filter
     public function __construct($column, $sign, $expression)
     {
         $column = trim($column);
-        $expression = is_array($expression) ? array_map('trim', $expression) : trim($expression);
         $this->column = $column;
         $this->sign = $sign;
         $this->expression = $expression;