Ignore proxy "Connection established" headers

The Squid proxy adds an extra header "HTTP/1.0 200 Connection established"
which libcurl (since 7.11.1) passes through unmodified.

Reference: http://curl.haxx.se/mail/lib-2005-10/0024.html
Merge pull request #12 from mrc/skip-proxy-headers

Ignore proxy "Connection established" headers