Explicitly stringify readpartial result

Consuming chunked stream may lead into unexpected `nil`s at any time of
consumption. Thus we should return `nil` only when `@socket` was closed.

Chunked responses are streamed in chunks. Each (full) chunk consist of:

  BYTES CRLF
  DATA CRLF

But socket does not care about consistency of those chunk, thus it's
common when we got only first part: `BYTES CRLF`. At this point we feed
this to http_parser which does not emits `on_data` event and our own
Parser `@chunk` ivar remains `nil` after previous flush.

This is no an ideal solution. Ideally we should relay explicitly on
http_parser `#finished?` flag instead. So this should be kept in mind
upon connection exctraction and refactoring.

Resolves #137