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

kalibrr/oauth2_proxy,bitly/google_auth_proxy,ploxiln/oauth2_proxy,snormore/oauth2_proxy,Shopify/google_auth_proxy,Shopify/google_auth_proxy,18F/oauth2_proxy,andyuc88/oauth2_proxy_azure,ploxiln/oauth2_proxy,bitly/oauth2_proxy,bitly/oauth2_proxy,snormore/oauth2_proxy,Shopify/google_auth_proxy,jehiah/oauth2_proxy,andyuc88/oauth2_proxy_azure,bitly/google_auth_proxy,kalibrr/oauth2_proxy,18F/oauth2_proxy,jehiah/oauth2_proxy