Fix query string being double-encoding over path proxy

Instead of trying to piece together the original URL and re-encode what
needs to be re-encoded, strip out the base from the original URL.

Fixes #6307.

test/unit/node/proxy.test.ts

@@ -208,6 +208,54 @@ describe("proxy", () => {
     const json = await resp.json()
     expect(json).toBe("ほげ")
   })
+
+  it("should not double-encode query variables", async () => {
+    const spy = jest.fn()
+    e.get("*", (req, res) => {
+      spy([req.originalUrl, req.query])
+      res.end()
+    })
+    codeServer = await integration.setup(["--auth=none"], "")
+    for (const test of [
+      {
+        endpoint: proxyPath,
+        query: { foo: "bar with spaces" },
+        expected: "/wsup?foo=bar+with+spaces",
+      },
+      {
+        endpoint: absProxyPath,
+        query: { foo: "bar with spaces" },
+        expected: absProxyPath + "?foo=bar+with+spaces",
+      },
+      {
+        endpoint: proxyPath,
+        query: { foo: "with-&-ampersand" },
+        expected: "/wsup?foo=with-%26-ampersand",
+      },
+      {
+        endpoint: absProxyPath,
+        query: { foo: "with-&-ampersand" },
+        expected: absProxyPath + "?foo=with-%26-ampersand",
+      },
+      {
+        endpoint: absProxyPath,
+        query: { foo: "ほげ ほげ" },
+        expected: absProxyPath + "?foo=%E3%81%BB%E3%81%92+%E3%81%BB%E3%81%92",
+      },
+      {
+        endpoint: proxyPath,
+        query: { foo: "ほげ ほげ" },
+        expected: "/wsup?foo=%E3%81%BB%E3%81%92+%E3%81%BB%E3%81%92",
+      },
+    ]) {
+      spy.mockClear()
+      const resp = await codeServer.fetch(test.endpoint, undefined, test.query)
+      expect(resp.status).toBe(200)
+      await resp.text()
+      expect(spy).toHaveBeenCalledTimes(1)
+      expect(spy).toHaveBeenCalledWith([test.expected, test.query])
+    }
+  })
 })
 
 // NOTE@jsjoeio