Allow concurrent websocket async ping and writes:

fix #271

This modifies the websocket stream implementation's composed
operations to allow caller-initiated asynchronous pings and
frame/message writes to take place at the same time.

include/beast/websocket/impl/ping.ipp

@@ -133,7 +133,7 @@ operator()(error_code ec, bool again)
             {
                 // suspend
                 d.state = 2;
-                d.ws.wr_op_.template emplace<
+                d.ws.ping_op_.template emplace<
                     ping_op>(std::move(*this));
                 return;
             }
@@ -188,7 +188,8 @@ operator()(error_code ec, bool again)
 upcall:
     if(d.ws.wr_block_ == &d)
         d.ws.wr_block_ = nullptr;
-    d.ws.rd_op_.maybe_invoke();
+    d.ws.rd_op_.maybe_invoke() ||
+        d.ws.wr_op_.maybe_invoke();
     d_.invoke(ec);
 }