Allow 1xx and 204 responses with Transfer-Encoding headers (#769)

Allow 1xx and 204 responses with Transfer-Encoding headers (#769)

HTTP servers MUST NOT send those header fields in those responses, but
some do, possibly because they compute the same basic headers for all
responses, regardless of the status code. Item 1 in RFC 7230 Section
3.3.3 is very clear about message framing in these cases. We have been
ignoring Content-Length under the same conditions since at least 2018.
We should be consistent and apply the same logic to Transfer-Encoding.

I also polished the Transfer-Encoding handling comment for clarity sake.