Fix keep-alive behavior

RFC 2616 section 8.1.1 states:

     HTTP implementations SHOULD implement persistent connections.

and section 14.10 defines the "close" connection option.

HTTP/1.0 does not use keep-alive unless the Connection
header is set to "Keep-Alive".
Fix keep-alive test

net/http only supports HTTP 1.1, so manually write the request.
Merge pull request #14 from titanous/fix-keepalive

Fix keep-alive behavior