remove remaining uses of sprintf() in the dav modules.

This is a regression in 2.3.7 introduced by r931434.
It calls sizeof() for a function parameter, which only returns the
pointer size, not the size of the char array. Thus the
"creationdate" property got truncated to three characters.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@987484 13f79535-47bb-0310-9956-ffa450edef68