Fix a bug where too much data was being returned. Allocated size instead of current size.
This might also have fixed a bug in GcsAccessorImpl::downloadObject().
When verifying the fix, do search for where get_response (the overload with two arguments) is called from and watch out for unintended consequences. Also make sure _response_buffer_written is always up to date.
The bugfix itself was rather banal once the problem was located. It may have uncovered other problems though. I will update the bug report.