Add `close_write` to `Connection`

This adds a `close_write` method to the connection class. This
will allow the Sanford server and AndSon client to close the
write stream of the socket after they have written a request or
response. The benefit is this alerts the remote socket that we
are done writing and they can begin reading.
Merge pull request #19 from redding/jcr-allow-closing-da-write

Add `close_write` to `Connection`