Sending HTTP/1.1 trailers

[seanmonstar]

Here's a list of pieces needed to make this work:

• Update proto::h1::Dispatcher::poll_write() so that when all data items are done (is None), check the body for trails poll_trailers.
  • It might be we want to add some state to the dispatcher like Wants::TRAILERS, if we received a request with TE: trailers. This could be useful to skip checking for trailers if the request never said it supports them.
  • It's likely that new state will need to be added, in case the data is done, but polling the trailers is Pending.
• Add proto::h1::Conn::write_trailers() after write_body(). The dispatcher would call this.
• Add proto::h1::Encoder::encode_trailers() that flattens the HeaderMap into a Buf.
  • A few unit tests for encoding trailers in the encode file.
• A couple tests, at least one in each of tests/client.rs and tests/server.rs that both sides can send trailers.

[seanmonstar]

@Xuanwo I've updated the issue above to include a list of things I think that needs to be done. If anything is missing, we can add more instructions here!