http: correct fencepost error when freeing socket

Per: https://github.com/joyent/node/pull/7260 /cc @othiym23

Original commit message: This only becomes apparent when using a small maxSockets for pipelining requests on keep-alive connection. Bug introduced in https://github.com/othiym23/node/commit/65f6f06a61c36630ee405a697b1d876208212874

This was originally submitted by @othiym23 back in March 2014 but never landed. It was reviewed by @indutny who requested that a test be included. This PR does not yet include that requested test