Fix false positives where some links would get flagged as "Unknown Error" and the log message would be "Empty reply from server".

Apparently some sites will temporarily IP-block any client that sends a HTTP request without a valid "Connection" header. Adding "Connection: close" and disabling CURL connection reuse fixes the problem. 

Keep-alive connections are not worth it in this case as the plugin already deliberate rate-limits requests. It would probably exceed the keep-alive timeout often.

git-svn-id: https://plugins.svn.wordpress.org/broken-link-checker/trunk@913393 b8457f37-d9ea-0310-8a92-e5e31aec5664