Disable HTTPClient to do keep-alive with server

In current API, HTTPClient instance is single-use and we should make
sure to close the opened socket (currently GC closes instead of it.)

It's better to create HTTPClient instance per API instance and reuse
it in the near future. Argument 'opts' and @-variables are immutable
so just set 'Date' header properly if we need (HTTPClient sets instead.)
Revert "Disable HTTPClient to do keep-alive with server"

This reverts commit 7ffe59bd12e0ffa5c7a7fe6f740479c773550ed7.
Disconnect keep-alive connection explicitly

For put request td-client-ruby uses HTTPClient. Each put request creates
HTTPClient instance (not recycled) and potentially it could cause many
unclosed connection. For now the connections are closed by GC.

Eventually put requests should share single HTTPClient instance.
Merge pull request #23 from treasure-data/topic_disable_httpclient_keepalive

Disconnect HTTPClient connection explicitly