force trailing zeroes in timestamp wire format

According to jwheare, ISO 8601 doesn't specify the behavior one way
or the other, and the server-time spec is also ambiguous, but this
is safest.

irc/client.go

@@ -28,7 +28,7 @@ import (
 const (
 	// IdentTimeoutSeconds is how many seconds before our ident (username) check times out.
 	IdentTimeoutSeconds  = 1.5
-	IRCv3TimestampFormat = "2006-01-02T15:04:05.999Z"
+	IRCv3TimestampFormat = "2006-01-02T15:04:05.000Z"
 )
 
 var (