Auto content-length setting

[guybedford]

We currently set content-type automatically during extract_body which is called in the request and response constructors.

According to the fetch spec, content-length is supposed to be set when initiating the request, , see here - https://fetch.spec.whatwg.org/#http-network-or-cache-fetch.

StarlingMonkey recently started setting content-length and content-type in this position, but I believe this is a bug as content-length should only be sent when initiating the response.

Should we be setting content-length further, or does XQD effectively handle this for us entirely?

Corrected from original post after verifying content-type is certainly being set correctly.