Support for async rendering with ejs

[TTPO100AJIEX]

According to #312 , ejs rendering with async: true did not work properly. This pull request does nothing but fixes this issue by awaiting the cachedPage before sending it back to the client if the rendering is occurring with ejs as an engine and async: true as an option. I am not very proficient at making pull requests, so feel free to correct me if I am doing something wrong. Thanks.

closes #312

Checklist

• [x] run npm run test and npm run benchmark
• [x] tests and/or benchmarks are included
• [x] documentation is changed or added
• [x] commit message and code follows the Developer's Certification of Origin and the Code of conduct