DbQuery: Return non-string args as is in `valueToTimestamp()`

fixes #4182

library/Icinga/Data/Db/DbQuery.php

@@ -251,7 +251,7 @@ class DbQuery extends SimpleQuery
     protected function valueToTimestamp($value)
     {
         // We consider integers as valid timestamps. Does not work for URL params
-        if (ctype_digit($value)) {
+        if (! is_string($value) || ctype_digit($value)) {
             return $value;
         }
         $value = strtotime($value);