Allow headers (if not already passed by other means) to be sent via
  WriteFile() since the foxisapi and other thunks seem to indicate that
  headers within the WriteFile() operation are expected to succeed.

  Also assure that all ->completion callbacks get the original size of
  the document.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@98533 13f79535-47bb-0310-9956-ffa450edef68