Use X-Auth-Request-Redirect request header in sign-in page

This is useful in Nginx auth_request mode, if a 401 handler is
configured to redirect to the sign-in page. As the request URL
does not reflect the actual URL, the value is taken from the
header "X-Auth-Request-Redirect" instead. Based on #247

oauthproxy.go

@@ -363,6 +363,9 @@ func (p *OAuthProxy) SignInPage(rw http.ResponseWriter, req *http.Request, code
 	rw.WriteHeader(code)
 
 	redirect_url := req.URL.RequestURI()
+	if req.Header.Get("X-Auth-Request-Redirect") != "" {
+		redirect_url = req.Header.Get("X-Auth-Request-Redirect")
+	}
 	if redirect_url == p.SignInPath {
 		redirect_url = "/"
 	}