Add ability to clear offset

Author: wilsonge

src/DatabaseQuery.php

@@ -560,6 +560,10 @@ public function clear($clause = null)
 				$this->limit = 0;
 				break;
 
+			case 'offset':
+				$this->offset = 0;
+				break;
+
 			case 'union':
 				$this->union = null;
 				break;