kv_vnode calling coverage_filter with 'all'

In [this code](https://github.com/basho/riak_kv/blob/e66f3ffdc637d0e2385301f74d984c0632029493/src/riak_kv_vnode.erl#L766), kv_vnode calls riak_kv_coverage:build_filter with the atom `all` as the first argument. The type spec for this function expects a `binary()`. Digging through the call stack, I couldn't find any cases where we pattern match on an atom or specifically the atom `all`. Any ideas?

cc @kellymclaughlin, who I believe wrote this code.