fix(router): securely check path for proper destination

[MegaManSec]

The router previously checked whether a path contained a route key. This check was insufficient, as it allowed a route configured as such:
'/admin': 'http://localhost:4002',
to be accessed to be accessible by requesting /some/public/path?q=/admin, which would match and route the request to localhost:4002.

If http-proxy-middleware is in front of a firewall which restricts access to /admin, this could have allowed unauthorized access to the admin route.

---

Given the following options:

const proxyOptions = {
  target: 'http://localhost:4001',
  changeOrigin: true,
  router: {
    '/admin': 'http://localhost:4002',
  },
  on: {
    proxyReq: (proxyReq, req, res) => {
      const targetHost = proxyReq.host;
      console.log(`[PROXY] Request for "${req.originalUrl}" is being routed to -> ${targetHost}`);
    },
  },
};

const proxy = createProxyMiddleware(proxyOptions);

it was previously possible to run curl 'http://localhost:3000/some/public/path?q=/admin to access the /admin endpoint.

How has this been tested?

Added testcases + manually.

• Bug fix (non-breaking change which fixes an issue)