header.go
func AppendNormalizedHeaderKey(dst []byte, key string) []byte {
	dst = append(dst, key...)
	normalizeHeaderKey(dst[len(dst)-len(key):], false)
	return dst
}
func AppendNormalizedHeaderKey(dst []byte, key string) []byte {
	dst = append(dst, key...)
	normalizeHeaderKey(dst[len(dst)-len(key):], false)
	return dst
}
// AppendNormalizedHeaderKey appends normalized header key (name) to dst
// and returns the resulting dst.
//
// Normalized header key starts with uppercase letter. The first letters
// after dashes are also uppercased. All the other letters are lowercased.
// Examples:
//
//   * coNTENT-TYPe -> Content-Type
//   * HOST -> Host
//   * foo-bar-baz -> Foo-Bar-Baz