server.go
func (ctx *RequestCtx) Value(key interface{}) interface{} {
	if keyString, ok := key.(string); ok {
		return ctx.UserValue(keyString)
	}
	return nil
}
func (ctx *RequestCtx) Value(key interface{}) interface{} {
	if keyString, ok := key.(string); ok {
		return ctx.UserValue(keyString)
	}
	return nil
}
// Value returns the value associated with this context for key, or nil
// if no value is associated with key. Successive calls to Value with
// the same key returns the same result.
//
// This method is present to make RequestCtx implement the context interface.
// This method is the same as calling ctx.UserValue(key)