set Path attribute in auth cookie to fix proxy auth in chrome

Authentication for private ports was broken in Chrome, and perhaps other
browsers too.  This was caused by the browser rejecting the `__Host-flow_auth`
cookie that was attempted to be set by data-plane-gateway.  The browser was
rejecting the cookie for a seemingly legitimate reason: the spec says it has to
be there.  I suspect that Firefox was more lax in this case because `Path=/` is
semantically equivalent to it not being set at all.  In any case, adding the
`Path=/` allows authenticated access to private ports to work properly in both
Chrome and Firefox.

proxy/redirect.go

@@ -61,6 +61,7 @@ func (h *authRedirectHandler) ServeHTTP(w http.ResponseWriter, req *http.Request
 		Value:    token,
 		Secure:   true,
 		HttpOnly: true,
+		Path:     "/",
 	}
 	http.SetCookie(w, cookie)
 	http.Redirect(w, req, origUrl, 307)