Replace io.Reader.Close() with drainAndClose.

Draining a reader involves reading all bytes until the io.EOF (or an error).
This is meant to optimize in favor of connection reuse which requires http
clients to read responses' bodies completely and close them.

This should deal with issue #17.
Merge pull request #18 from cookieo9/conn_drain

Replace io.Reader.Close() with drainAndClose.