Trailing dashes in Desktop keys breaks merging

If a translatable Desktop entry ends in a dash (`-`), intltool-merge will generate duplicate entries of the original key and value rather than translated versions with language suffix. This both leads to actually untranslated output, and to invalid .desktop files (as the spec forbids duplicate entries).

Attached patch fixes it and adds a test case. The patch is made on top of https://bugs.launchpad.net/intltool/+bug/1330088/comments/2 because the test case conflict, but I can provide a standalone version if you don't want to merge the other patch just yet.