fix small bug for HTTP IO write access

git-svn-id: svn://dev.exiv2.org/svn@3135 b7c8b350-86e7-0310-a4b4-de8f6a8f16a3
msvc2005: I think this is working perfectly on 2005.  See ReadMe.txt for caution about 2010.

git-svn-id: svn://dev.exiv2.org/svn@3136 b7c8b350-86e7-0310-a4b4-de8f6a8f16a3