Correctly pass kwargs through to wheel.call_func

The change in 1de6204 broke what ``format_call()`` expects in ``wheel.Wheel().call_func()``. That said I'm not sure what nested kwargs are or where they're used so this commit may very well break something elsewhere. In my simple tests ``clear_load`` never contained a ``kwarg`` value so I'm submitting this change for review.

@thatch45 ping