Get byterange requests working with data that is generated by a handler

that does not add a content-length.  For example, mod_autoindex doesn't
set a content-length, but the byterange filter requires one.  We fix this
by computing the content-length in the byterange filter.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@87389 13f79535-47bb-0310-9956-ffa450edef68