Fix keep-alive connection closing for async

This commit fixes an issue where the async server crashes when the client closes a keep-alive connection. The exception indicates it's a parse error, when in fact it's the server hitting an EOF.

The solution involves throwing an exception in the `Request` module. This may or may not play well with lwt. Perhaps a better solution, but more pervasive change, would be to introduce a type with three variants: `Ok`, `Eof`, and `Error`.

Related to avsm/ocaml-cohttp#85.