Bug 52400: fix handling some types of TNEF files, make HMEFMessage.HEADER_SIGNATURE int as it is handled as int everywhere, move .dat files to HMEF sample dir, cover some border cases in HMEFMessage, add another .dat sample file

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1538353 13f79535-47bb-0310-9956-ffa450edef68