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

Restore `kind_of(...)` to match using `kind_of?` rather than `===`

Fixes #744.
I think this would be better phrased as:

```
* Restore `kind_of(x)` argument matcher to use `arg.kind_of?(x)`
  rather than `x === arg` as was the case in RSpec 2.
```