initializing Bins with bins=1 breaks figure

[kecnry]

Describe the bug
Similar to #1614, initializing a Bins mark with bins=1 breaks the figure (although oddly a resize event seems to resuscitate it).

To Reproduce

import bqplot

fig = bqplot.Figure()
fig.axes = [bqplot.Axis(scale=bqplot.LinearScale(), label='x'),
            bqplot.Axis(scale=bqplot.LinearScale(), orientation='vertical', label='y')]

orig_bins = 1  # breaks figure
orig_bins = 2  # does NOT break figure

bins = bqplot.Bins(scales={'x': fig.axes[0].scale,
                            'y': fig.axes[1].scale},
                    sample=[0, 1, 2, 3], bins=orig_bins,
                    colors=['gray'], size=12)

fig.marks = [bins]

fig

and then later, revising the sample/bins:

bins.sample = np.random.random(20)
bins.bins = 10

if orig_bins != 1, the figure updates as expected. If orig_bins == 1, there do seem to be some updates, but the figure is not correct until the widget is resized.

with orig_bins = 1 and then updating bins.sample and bins.bins:

after resizing the figure: