Update fetch implementation to solve forced lowercase header property names

[eksperts]

The fetch implementation we currently have seems to be somewhat outdated, leading to issues like this (forced lowercase header property names), while the standard has changed around that particular thing.

Prior to updating, needs extensive testing, since other changes might turn out to be backwards-incompatible.

Edit: this ticket is about fixing the fetch implementation around the header normalisation, so that property names are not forced to lowercase, according to the spec changes discussed in the linked ticket.

[fusebuild]

The forum thread Missing Header posted by papakay19@gmail.com was linked to this issue.

[kusma]

@eksperts: Would you mind filling in an exhaustive list of the things you know needs fixing? Otherwise this will quickly end up as one of those tickets that can never be closed...

[eksperts]

@kusma this lowercase-headers thing is the only one I'm aware of. Changing this ticket to "fix this one thing" then.

If we had the time, someone from engineering or features could evaluate what other changes have been implemented since our fetch implementation snapshot. Such task needs prioritisation, and I leave that to you.

[kusma]

@eksperts: Thanks.