func (u *Username) GetFrom(m *Message) error {
	return (*TextAttribute)(u).GetFromAs(m, AttrUsername)
}
GetFrom gets USERNAME from message.
package stun
func NewUsername(username string) Username {
	return Username(username)
}
type Username []byte
func (u Username) String() string {
	return string(u)
}
const maxUsernameB = 513
func (u Username) AddTo(m *Message) error {
	return TextAttribute(u).AddToAs(m, AttrUsername, maxUsernameB)
}