mod_lua: If the first yield() of a LuaOutputFilter returns a string, it should
be prefixed to the response as documented.

Also, don't put empty heap buckets in the brigade if a yield() is called with 
no string.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1519492 13f79535-47bb-0310-9956-ffa450edef68