Disable autoloader call from class_exists() occurences to improve performance

Note: The Driver libary tests seem to depend on that, so one occurence in CI_Loader is left until we resolve that.
diff --git a/system/database/DB.php b/system/database/DB.php
index d910474..83d9733 100644
--- a/system/database/DB.php
+++ b/system/database/DB.php
@@ -149,7 +149,7 @@
 	if ( ! isset($query_builder) OR $query_builder === TRUE)
 	{
 		require_once(BASEPATH.'database/DB_query_builder.php');
-		if ( ! class_exists('CI_DB'))
+		if ( ! class_exists('CI_DB', FALSE))
 		{
 			/**
 			 * CI_DB
@@ -162,7 +162,7 @@
 			class CI_DB extends CI_DB_query_builder { }
 		}
 	}
-	elseif ( ! class_exists('CI_DB'))
+	elseif ( ! class_exists('CI_DB', FALSE))
 	{
 		/**
 	 	 * @ignore