Redefining this matcher generates warnings:

```
/Users/myron/code/rspec-dev/repos/rspec-expectations/lib/rspec/matchers/dsl.rb:8: warning: method redefined; discarding old custom_include
```

The other custom matchers below (`have_string_length` and `be_a_clone`) have the same problem.  They were defined in-line before because they were only used in one spot.  At this point, we should move them into `spec/support/matchers.rb`.