Add description to support RSpec's attribute of subject

This is a patch to support RSpec's attribute of subject function.

When I use the function, the matching works fine, but I get the following message.

> should When you call a matcher in an example without a String, like this:
> 
> specify { object.should matcher }
> 
> or this:
> 
> it { should matcher }
> 
> RSpec expects the matcher to have a #description method. You should either
> add a String to the example this matcher is being used in, or give it a
> description method. Then you won't have to suffer this lengthy warning again.

The explanation of "attribute of subject" is found in the below url.
https://www.relishapp.com/rspec/rspec-core/v/2-12/docs/subject/attribute-of-subject