Restrict / review `Transfer-Encoding` header rendering

Since #3754, we only support Transfer-Encoding: chunked headers and no other ones. The reasoning is that akka-http is implementing the "Transfer" part of HTTP. Leaving parsing or providing custom Transfer-Encoding headers open to the user will lead to confusion and bugs.

We should check that:

• The Transfer-Encoding model could make sure that the latest entry is always chunked (see RFC), though that would change the error reporting for parsing as now implemented with #3754
• User-provided Transfer-Encoding headers will always be ignored