implement SetTimeout without touching http.Client.Timeout

[okhowang]

currently SetTimeout just modify http.Client.Timeout

func (c *Client) SetTimeout(timeout time.Duration) *Client {
	c.httpClient.Timeout = timeout
	return c
}

when we create resty.Client with NewWithClient. it do side effect if we use http.Client in other code.
maybe we should keep timeout in resty.Client, and do context.WithTimeout before every request?

[yunduan16]

+1

[aadog]

#574

[aadog]

I implemented session pattern, you can use