expect({a: 1}).to.have.only.key('a') fails

It appears that `have` and `only` cannot be swapped:

``` js
expect({a: 1}).to.only.have.key('a') // this works
expect({a: 1}).to.have.only.key('a') // this doesn't
```

I would expect the second to work, given the documentation, but it fails.

Reproduce both assertion chains shown in the issue, then search the JavaScript assertion implementation for how `have`, `only`, and `key` are handled. Done means `expect({a: 1}).to.have.only.key('a')` passes consistently with the documented ordering, while the existing working form remains intact.

Written by the indexing model from the issue text.