Form helper _attributes_to_string() micro-optimization

As $attributes should be most of the times an array, let's save an is_string() call.
diff --git a/system/helpers/form_helper.php b/system/helpers/form_helper.php
index 6fca73f..5ba5b55 100644
--- a/system/helpers/form_helper.php
+++ b/system/helpers/form_helper.php
@@ -931,11 +931,6 @@
 	 */
 	function _attributes_to_string($attributes)
 	{
-		if (is_string($attributes))
-		{
-			return ($attributes === '' ? '' : ' '.$attributes);
-		}
-
 		if (is_object($attributes))
 		{
 			$attributes = (array) $attributes;
@@ -953,6 +948,11 @@
 			return $atts;
 		}
 
+		if (is_string($attributes))
+		{
+			return ($attributes === '' ? '' : ' '.$attributes);
+		}
+
 		return FALSE;
 	}
 }