func (exts Extensions) Get(key string) string {
	if exts == nil {
		return ""
	}
	v := exts[key]
	if len(v) == 0 {
		return ""
	}
	return v[0]
}
Get gets the first value associated with the given key.
It is case sensitive;
If there are no values associated with the key, Get returns "".
package auth
type Extensions map[string][]string
func (exts Extensions) Add(key, value string) {
	exts[key] = append(exts[key], value)
}
func (exts Extensions) Set(key, value string) {
	exts[key] = []string{value}
}
func (exts Extensions) Del(key string) {
	delete(exts, key)
}