authenticate: refactor middleware, logging, and tests (#30)

- Abstract remaining middleware from authenticate into internal.
- Use middleware chaining in authenticate.
- Standardize naming of Request and ResponseWriter to match std lib.
- Add healthcheck / ping as a middleware.
- Internalized wraped_writer package adapted from goji/middleware.
- Fixed indirection issue with reverse proxy map.

authenticate/providers/providers.go

@@ -258,7 +258,7 @@ func (p *ProviderData) RefreshAccessToken(refreshToken string) (string, time.Dur
 		log.Error().Err(err).Msg("authenticate/providers.RefreshAccessToken")
 		return "", 0, err
 	}
-	return newToken.AccessToken, newToken.Expiry.Sub(time.Now()), nil
+	return newToken.AccessToken, time.Until(newToken.Expiry), nil
 }
 
 // Revoke enables a user to revoke her token. If the identity provider supports revocation