Workaround Curl bug where Proxy headers are not included in header_size

Versions of Curl prior to 7.30.0 do not properly account for additional headers injected via proxies when calculating header_size. This is a quick workaround based on:
https://github.com/google/google-api-php-client/issues/117

This detects the commonly added proxy header and removes the length so that it returns correctly. It has no effect if the curl version is higher than 7.30.0.

Ref:
https://github.com/google/google-api-php-client/issues/117
https://bugs.php.net/bug.php?id=63894
http://curl.haxx.se/mail/lib-2013-04/0189.html