Add description for object to rspec support

Necessary for https://github.com/rspec/rspec-mocks/pull/819
``` ruby
RSpec::Matchers.define :matcher_with_no_description do
  match { true }
  undef description
end

it 'has no description but is still a matcher' do
  expect(matcher_with_no_description).not_to respond_to(:description)
  expect(RSpec::Support.is_a_matcher?(matcher_with_no_description)).to eq true
end
```