Use binary_to_atom/2 instead of binary_to_existing_atom/2.

There's a possibility that the riak_dt code that contains the set
operation atoms hasn't been loaded when
riak_kv_crdt_json:set_op_from_json/1 is invoked, meaning
binary_to_existing_atom/2 will raise a badarg. We are already checking
the incoming operation as a binary via a guard on the relevant
function clause, so it is safe to use binary_to_atom/2 instead.
Merge pull request #933 from basho/bugfix/sdc/http-crdt-badarg

Use binary_to_atom/2 instead of binary_to_existing_atom/2.