Improve spec compliance of transmit request body

[gterzian]

Spec: https://fetch.spec.whatwg.org/#concept-request-transmit-body

See various TODOs at https://github.com/servo/servo/pull/25873/files#diff-ae0ff1fd98b06dfb13baf427cdffc28aR171

I think this will require storing a Trusted<Request> on the TransmitBodyConnectHandler(in addition to the currently stored Trusted<ReadableStream>), and then queue the various tasks for things like "process end of request body".

Those currently are actually implemented as placeholders, see

servo/components/script/fetch.rs

Line 203 in 0281ace

fn process_request_body(&mut self) {

However I think the process_request_body and process_request_eof methods on FetchContext and FetchResponseListener might have to be removed in favor of the new mechanism described here where you'd just queue the tasks using the actual Request and the TransmitBodyConnectHandler.

[gterzian]

Actually we probably shouldn't try to queue tasks for this, see the discussion over at whatwg/fetch#536