Fix typo on return value

seems like there was a typo on this. Without this, shopify tests fail with:
```
Error:
test_#foo
NoMethodError: undefined method `cast' for nil:NilClass
    /home/ubuntu/shopify/vendor/bundle/ruby/2.1.0/bundler/gems/activerecord-typedstore-66448f4c2bcc/lib/active_record/typed_store/typed_hash.rb:52:in `cast_value'
```