Response headers added by proxy servers missing in CURLINFO_HEADER_SIZE

Often times proxy servers add their own headers (like `HTTP/1.0 200 Connection established`) to responses on their way back to the client. The size of these headers was not taken into account when calculating `CURLINFO_HEADER_SIZE` before the change from this pull request.

The problem can be reproduced quite easily using the test program from https://gist.github.com/mj/5102778. In our case the proxy server at `proxy:8080` was the default Squid that ships with Debian stable.