Show the final request headers sent to the server

[sirdarckcat]

See #134 - the TL;DR is that the Fetch API exposes only the headers available early on in the request flow, which might be different from the ones utlimately sent to the server (Accept-Encoding and Accept-Language for example).

It would be nice if, although we can't show them before they are sent, if we added them once we can see them (they should be available in the Network domain).