Add missing documentation information

[ratchetsniper2]

I found a few points that could be added to the documentation:

1. HttpRequest.forEach: Specify that both the key and value are in lower case.
2. HttpResponse.onData: Specify that, like onAborted, it must be attached to the HttpResponse before any asynchronous operation, otherwise request body data will be dismissed.
3. HttpResponse.writeStatus: Specify that it must be called before any write operation (writeHeader/write/end*) otherwise the default status "200 OK" will be used.