Don't set the slug when creating a new resource without a subject

Without this patch I see this error:

```
     NoMethodError:
       undefined method `strip' for nil:NilClass
     # /Users/justin/workspace/ldp/lib/ldp/client/methods.rb:70:in `post'
     # /Users/justin/workspace/ldp/lib/ldp/resource/rdf_source.rb:24:in `create'
     # /Users/justin/workspace/ldp/lib/ldp/orm.rb:43:in `block in create'
     # /Users/justin/workspace/ldp/lib/ldp.rb:32:in `instrument'
     # /Users/justin/workspace/ldp/lib/ldp/orm.rb:40:in `create'
```