It’s possible to send header blocks from hyper-as-server that hyper-as-client will reject #262

Picking up a comment from #261:

We do a series of checks in the validate_headers pipeline for incoming header blocks, in particular:

• All fields should be lowercase.
• A Transfer-Encoding header must have value "trailers"
• No blocks with the Connection header.
• No duplicate pseudo-header fields and/or pseudo-header fields coming after an ordinary header field.
• No header blocks with bad :authority/Host headers.

But we don’t enforce all these rules on header blocks that we send, which suggests that it’s possible to send a header block with hyper that we’d reject if we received. Which seems odd.

(1) is achieved by the _lowercase_header_names pipeline, and I’m adding (5) in #261. I think we should be enforcing (2)-(4) on header blocks that we send as well.

Does that seem sensible?