Correct cURL version containing the proxy header miscalculation fix

Source: http://curl.haxx.se/changes.html

Search for "corrected proxy header response headers count."
Fix cURL proxy header response miscalculation compensation logic

This directly compensates for the cURL problems in versions < 7.30.0 while
retaining backwards compatibility with the existing API.

The previous behavior would subtract the proxy header size even though cURL
versions less than 7.30.0 did not take them into account in the first place,
resulting in calculated header sizes that were too small.