shrpx: Fix WINDOW_UPDATE may block until SpdyUpstream::send()

spdy_data_read_callback in SpdyDownstreamConnection calls
SpdyUpstream::resume_read() which submits WINDOW_UPDATE, but after
that they are not call SpdyUpstream::send(). This means that if no
pending outgoing data in upstream, then WINDOW_UPDATE is blocked until
SpdyUpstream::send() from somewhere. This change adds
SpdyUpstream::send() to resume_read() so that WINDOW_UPDATE is not
blocked.

src/shrpx_spdy_downstream_connection.cc

@@ -190,6 +190,11 @@ ssize_t spdy_data_read_callback(spdylay_session *session,
           // In this case, downstream may be deleted.
           return SPDYLAY_ERR_DEFERRED;
         }
+        // Check dconn is still alive because Upstream::resume_read()
+        // may delete downstream which will delete dconn.
+        if(sd->dconn == 0) {
+          return SPDYLAY_ERR_DEFERRED;
+        }
         if(evbuffer_get_length(body) == 0) {
           return SPDYLAY_ERR_DEFERRED;
         }

src/shrpx_spdy_upstream.cc

@@ -898,7 +898,7 @@ int SpdyUpstream::resume_read(IOCtrlReason reason, Downstream *downstream)
       window_update(downstream);
     }
   }
-  return 0;
+  return send();
 }
 
 } // namespace shrpx