proxy.ts: Do not always rewrite redirects against the base path

This breaks --proxy-path-passthrough

However, we still need this when that code is disabled as many apps will
issue absolute redirects and expect the proxy to rewrite as appropriate.

e.g. Go's http.Redirect will rewrite relative redirects as absolute!
See https://golang.org/pkg/net/http/#Redirect

src/node/proxy.ts

@@ -9,6 +9,8 @@ proxy.on("error", (error, _, res) => {
 })
 
 // Intercept the response to rewrite absolute redirects against the base path.
+// Is disabled when the request has no base path which means --proxy-path-passthrough has
+// been enabled.
 proxy.on("proxyRes", (res, req) => {
   if (res.headers.location && res.headers.location.startsWith("/") && (req as any).base) {
     res.headers.location = (req as any).base + res.headers.location

src/node/routes/pathProxy.ts

@@ -28,8 +28,10 @@ router.all("/(:port)(/*)?", (req, res) => {
     throw new HttpError("Unauthorized", HttpCode.Unauthorized)
   }
 
+  if (!req.args["proxy-path-passthrough"]) {
     // Absolute redirects need to be based on the subpath when rewriting.
     ;(req as any).base = `${req.baseUrl}/${req.params.port}`
+  }
 
   proxy.web(req, res, {
     ignorePath: true,