`h2` returns `RST_STREAM` frames with the `PROTOCOL_ERROR` bit set as a
response to many types of errors in client requests. Many of those cases,
when handled by an HTTP/1 server such as the one used in `hyper`, would
result in an HTTP 400 Bad Request response returned to the client rather
than a TCP reset (the HTTP/1 equivalent of a `RST_STREAM`). As a
consequence, a client will observe differences in behaviour between
HTTP/1 and HTTP/2: a malformed request will result in a 400 response when
HTTP/1 is used whereas the same request will result in a reset stream
with `h2`.

Make `h2` reply a `HEADERS`+`400`+`END_STREAM` frame followed by a
`RST_STREAM`+`PROTOCOL_ERROR` frame to all the `malformed!()` macro
invocations in `Peer::convert_poll_message()` in `src/server.rs`.

The `Reset` variant in the `h2::proto::error::Error` Enum now contains an
`Option<http::StatusCode>` value that is set by the `malformed!()` macro
with a `Some(400)`. That value is propagated all the way until
`h2::proto::streams::send::Send::send_reset()` where, if not `None`, a
`h2::frame::headers::Headers` frame with the status code and the
`END_STREAM` flag set will now be sent to the client before the
`RST_STREAM` frame.

Some of the parameters passed to `send_reset()` have been grouped into a
new `SendResetContext` Struct in order to avoid a
`clippy::too_many_arguments` lint.

Tests where malformed requests were sent have been updated to check that
an additional  `HEADERS`+`400`+`END_STREAM` frame is now received before
the `RST_STREAM + PROTOCOL_ERROR` frame.

This change has been validated with  other clients like `curl`, a custom
ad-hoc client written with `python3+httpx` and `varnishtest`.