mod_proxy_html: follow up to r1599012.

To determine whether or not HTML data are lower than 4 bytes, use a retain
buffer rather than assuming that all should be contained in a single bucket
with the next one being EOS (if any).



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1819969 13f79535-47bb-0310-9956-ffa450edef68