matchers.go
func NewHeader(m map[string]string) Header {
	for k, v := range m {
		delete(m, k)
		m[http.CanonicalHeaderKey(k)] = v
	}
	return Header(m)
}
func NewHeader(m map[string]string) Header {
	for k, v := range m {
		delete(m, k)
		m[http.CanonicalHeaderKey(k)] = v
	}
	return Header(m)
}
// Header ---------------------------------------------------------------------
// NewHeader returns a header matcher, converting keys to the canonical form.