Add "Connection: close" to the HTTP request header, which tells HTTP/1.1

Add "Connection: close" to the HTTP request header, which tells HTTP/1.1
servers to close the connection after completion of the response.  This
should be compatible with HTTP/1.0, as HTTP/1.0 allows for custom header
fields which should be ignored if not recognized by the server.  With
some server configurations, the server would keep the connection alive
without this header line (despite the plugin requesting HTTP/1.0),
resulting in the plugin running into either the server's or it's own
timeout (reported by Roman Schliessmeyer and Jesse Morgan via IRC).

git-svn-id: https://nagiosplug.svn.sourceforge.net/svnroot/nagiosplug/nagiosplug/trunk@1759 f882894a-f735-0410-b71e-b25c423dba1c