Don't send a close frame in response to receiving one, if we already sent a close frame.

lib/websocket/driver/hybi.rb

@@ -246,7 +246,7 @@ module WebSocket
       end
 
       def shutdown(code, reason)
-        frame(reason, :close, code)
+        frame(reason, :close, code) if @ready_state < 2
         @frame = @message = nil
         @ready_state = 3
         @stage = 5

spec/websocket/driver/hybi_spec.rb

@@ -457,6 +457,11 @@ describe WebSocket::Driver::Hybi do
       it "changes the state to :closed" do
         expect(driver.state).to eq :closed
       end
+
+      it "does not write another close frame" do
+        expect(socket).not_to receive(:write)
+        driver.parse [0x88, 0x04, 0x03, 0xe9, 0x4f, 0x4b].pack("C*")
+      end
     end
   end