Add DefaultRedirectPolicy

[rosahaj]

There's currently no easy and intuitive way to change a client's redirect policy back to the default redirect policy.

For example: Let's say someone wants to send a GET request to a given URL, but is only interested in the Location header and the cookies of the response. They can use c.SetRedirectPolicy(req.NoRedirectPolicy()) to disable redirects and manually grab the Location header, but there's no simple way to "reset" the redirect policy back to the default one.

They could go through req's source code until they find the following function, only to realize that they can achieve the same thing by setting the redirect policy to req.MaxRedirectPolicy(10):

req/client.go

Lines 324 to 332 in 24b0c84

	func (c *Client) defaultCheckRedirect(req *http.Request, via []*http.Request) error {
	if len(via) >= 10 {
	return errors.New("stopped after 10 redirects")
	}
	if c.DebugLog {
	c.log.Debugf("<redirect> %s %s", req.Method, req.URL.String())
	}
	return nil
	}

This PR introduces req.DefaultRedirectPolicy(), which is essentially just an alias for req.MaxRedirectPolicy(10), but greatly improves the developer experience in scenarios where someone wants to set a specific redirect policy, send out a couple of requests and then revert back to the default one.

For relevant documentation changes, see imroc/req-website#239 (comment)

[imroc]

Thanks