Bug fix in `PlotlyDict.to_graph_objs`.

Before:
-------

`Scatter(xaxis='x')` --> `Unpack Error`

Because `'xaxis'` is a key that *could* be referencing a `PlotlyDict`,
we assumed that it was a subclass of `dict` and could therefore be
unpacked with the `**` operator.

After:
------
`Scatter(xaxis='x')`
`{'type': 'scatter', 'xaxis': 'x'}`