Add description for object to rspec support

Necessary for https://github.com/rspec/rspec-mocks/pull/819
What's the best way to define a matcher without `#description`? I've got a spike that looks like this:

```
  3         it "returns the object for a matcher that does not have a description" do
  4           matcher = fake_matcher(nil)
  5
  6           matcher.instance_eval do
  7             undef :description
  8           end
  9
 10           expect(RSpec::Support.rspec_description_for_object(matcher)).to be matcher
```

Thoughts?