Adds X-Riak-Deleted where missing

In the case of a single object being returned which is a
tombstone and the case where a single sibling has been
requested via a vtag and it is a tombstone, the
X-Riak-Deleted header will now be added and
returned.

Ideally both these would return 204 rather than 404 and 200
respectively but doing so would have breaking consequences
for clients.
Reverted to checking for string instead of atom

The change in #532 makes this a string again ("true")
Merge pull request #520 from basho/gh518-http-tombstone-headers

Adds X-Riak-Deleted where missing