Fix problem reported by Taketo Kabe <kabe@sra-tohoku.co.jp>

where HEAD response headers were being repeated twice for
files greater than 32K bytes (4*AP_MIN_BYTES_TO_WRITE). This
problem in the http_header filter was exposed by the recent rewrite
of the content_length filter.
[Taketo Kabe, Bill Stoddard]

PR: 8037


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@89676 13f79535-47bb-0310-9956-ffa450edef68