Restore kind_of to match using kind_of? rather than ===

Restore `kind_of(...)` to match using `kind_of?` rather than `===`

Fixes #744.
This docstring is confusing, because the example has nothing to do with the fact that `===` has been overriden -- it has to do with the fact that `kind_of?` has been overriden.  After all, `===` is sent to the argument passed to `kind_of` (`Numeric`, in your case) and in your example, `Numeric.===` has not been overriden.

I think it would be better as:

``` ruby
it 'matches arguments that have defined `kind_of?` to return true' do
  fix_num = double(:kind_of? => true)
  expect(a_double).to receive(:random_call).with(kind_of(Numeric))
  a_double.random_call(fix_num)
end
```