Fix --always-copy

a slight makeover of PR #496 for fixing #495 

It fixes the 3rd commit from #496:  https://github.com/vldmit/virtualenv/commit/ec05a4ea8732a3b75235b048d7809040e07c44a3. that was mistakenly using `copyfileordir`, instead of `copyfile`.   

Despite some confusion in the conversation in #496, the idea of the 3rd commit *was* necessary to make the `--always-copy` work correctly for the "posix_local" scheme (which e.g. travis uses for py27 tests.  to the see the failure w/o this change, see  https://travis-ci.org/pypa/virtualenv/jobs/15023174 )

also, made some tweaks to the test.