stream: support raw Response content

[wenchy]

refer: https://requests.readthedocs.io/en/latest/user/quickstart/#raw-response-content

In the rare case that you’d like to get the response Body stream (io.ReadCloser) from the server, you can access r.Raw(). If you want to do this, make sure you set stream as true in your initial request. Once you do, you can do this:

resp, err := requests.Get("http://example.com", requests.Stream(true))

Implementation

add option:

type Options struct {
	// ...
	Stream bool
}

// Stream sets whether need streamed response from the server, you can access resp.Raw(). 
func Stream(yes bool) Option {
	return func(opts *Options) {
		opts.Stream = yes
	}
}