Is there an existing issue for this?

• I have searched the existing issues

Current Behavior

FormatMessageType returns the empty string for an unknown message type.

Expected Behavior

FormatMessageType should return the decimal representation of the type when the type is unknown.

Steps To Reproduce

 fmt.Println(websocket.FormatMessageType(7777))

Actual output: blank line
Expected output: 7777

Anything else?

Cc @rfyiamcool