Add failing key tx test

Change the default key transformation to only allow binaries. This will
make it have the same assumption that previous user defined key transforms
have had since they were created. It will fail if it gets passed a
{tombstone, Key} tuple, which is the problem being fixed here.
Run a merge with completely expired files, one of which contains a
tombstone. This will trigger the path that passes the {tombstone, Key}
instead of just the Key to the key transformer. This will fail with a
function_clause error as expected.
Skip tombstones on expired file merge

When merging completely expired files, we fold over them issuing
conditional keydir deletes on the entries of the file. That way, if any
of them are still present in the keydir, they will be deleted now
instead of the usual lazy delete on read, when we detect they are
expired.

Previously, the code was not accounting for the new {tombstone, Key}
return format when folding over the hintfile. A recent change in 1.7.0
added a tombstone bit flag to hintfiles, so now we can tell we are
dealing with a tombstone.  The tuple was being passed whole to the user
defined key transformation function, causing a crash. With this change,
we simply skip over tombstones. It makes no sense to try to delete them
from the keydir.
Merge pull request #186 from basho/bugfix/key-transform-crash

Bugfix/key transform crash

Reviewed-by: jonmeredith