Return an error at EOF while reading the PROXY protocol header

[armaxri]

Fixes #580.

read_proxy_header loops forever when the peer closes the connection before the header's
newline arrives: peek returns Ok(0) at EOF, no newline is found, and the zero-length
read in the None branch returns Ok(0) too.

peek only returns Ok(0) at EOF (the buffer is never empty), so this checks for it right
after the peek and returns ProxyError::ReadError with io::ErrorKind::UnexpectedEof. The
spawned header task then logs "proxy protocol read error" and ends, like for every other
header error. Behaviour for connections that do send a header is unchanged.

Two tests cover EOF before any byte and EOF after a partial header; both used to hang and now
return the error (they are bounded by a 5 s timeout so a regression fails instead of hanging).