jwt.go
func PeekHeaderField(buf []byte, field string) (string, error) {
	return peekField(buf, field, tokenPositionHeader)
}
func PeekHeaderField(buf []byte, field string) (string, error) {
	return peekField(buf, field, tokenPositionHeader)
}
// PeekHeaderField extracts the specified field from the serialized JWT buf's
// header. An error will be returned if the field is not present in the decoded
// header.