this `lists:keydelete` causes a problem because the props returned from `validate` are the ones stored. if I change `riak_core_bucket_type:defaults/0` [1] to `{allow_mult, false}` then I can create a bucket-type with a `datatype` but `allow_mult=false`:

```
$ dev/dev1/bin/riak-admin bucket-type create crdt1 '{"props": {"datatype": "counter"}}'
...
$ dev/dev1/bin/riak-admin bucket-type status crdt1 | grep "datatype\|allow_mult"
datatype: counter
allow_mult: false
```

Was the intention to not validate `allow_mult` again? If I change this line to `validate(Props, Valid, Errors)` and leave the change to `riak_core_bucket_type:defaults/0` then I get a validate error (as expected) when running the creation command above but if I specify `allow_mult: true` in the creation command everything still works fine.

[1] https://github.com/basho/riak_core/blob/develop/src/riak_core_bucket_type.erl#L129