Fix re-export module Frame on Websocket_async (#110)

Author: kkazuo

async/websocket_async.ml

@@ -20,6 +20,8 @@ open Core
 open Async
 open Cohttp
 
+module Frame = Websocket.Frame
+
 module Async_IO = Websocket.Make(Cohttp_async.Io)
 open Async_IO
 

async/websocket_async.mli

@@ -28,6 +28,8 @@ open Websocket
 open Core
 open Async
 
+module Frame = Websocket.Frame
+
 val client :
   ?name:string ->
   ?extra_headers:Cohttp.Header.t ->