README: fix nginx auth_request example for requests with body
README: fix nginx auth_request example for requests with body

Nginx never sends the body with the auth_request sub-request, but
keeps the original Content-Length header by default. Without some
config tweaks, this results in the request to /oauth2/auth hanging.

18F/oauth2_proxy,Shopify/google_auth_proxy,jehiah/oauth2_proxy,bitly/google_auth_proxy,Shopify/google_auth_proxy,kalibrr/oauth2_proxy,Shopify/google_auth_proxy,bitly/oauth2_proxy,ploxiln/oauth2_proxy,jehiah/oauth2_proxy,bitly/oauth2_proxy,kalibrr/oauth2_proxy,ploxiln/oauth2_proxy,18F/oauth2_proxy,bitly/google_auth_proxy