fix: restore express prototypes so injecting doesn't break http requests

[gunjam]

I'm attempting to port an internal express app test framework to use light-my-request because it hugely increases performance (thanks!), but some of the framework's plugins still need to make HTTP requests.

Currenlty the first inject() call will permenantly modify the app's request and response prototypes, making subsequent HTTP requests crash the server - this change fixes that by caching them and setting the back to the originals.

Ta

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct