Revert a regression introduced in the recent overhauls of mod_isapi.

PR 40470 points out that we no longer handle ate == headlen, where
there is a headers-only response from the isapi app to the server.

This patch restores the previous behavior of pending those headers
until some amount of body response is ready to be sent to the client.

Backports: 483613


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.0.x@483615 13f79535-47bb-0310-9956-ffa450edef68