context_header.go
func (c headerCtx) Headers() map[string]string {
	if h := c.headers(); h != nil {
		return h.reqHeaders
	}
	return nil
}
func (c headerCtx) Headers() map[string]string {
	if h := c.headers(); h != nil {
		return h.reqHeaders
	}
	return nil
}
// Headers gets application headers out of the context.