partial always use partial buffering if send incomplete

After "rerelease"

https://github.com/warmcat/libwebsockets/issues/392#issuecomment-170003294



Since we introduced partial buffering a long while ago,
user code shold never see partial sends and very few
user callbsck attempt to deal with them.

Let's just eliminate the whole concept of user callback
partial send handling under any circumstances.

Signed-off-by: Andy Green <andy.green@linaro.org>

lib/output.c

@@ -171,16 +171,6 @@ handle_truncated_send:
 		/* what we just sent went out cleanly */
 		return n;
 
-	if (n && wsi->u.ws.clean_buffer)
-		/*
-		 * This buffer unaffected by extension rewriting.
-		 * It means the user code is expected to deal with
-		 * partial sends.  (lws knows the header was already
-		 * sent, so on next send will just resume sending
-		 * payload)
-		 */
-		 return n;
-
 	/*
 	 * Newly truncated send.  Buffer the remainder (it will get
 	 * first priority next time the socket is writable)