Contour plot color not encoded with Maplotlib

It works if `c/color` is set to `'air'` but that shouldn't be required.

```python
import hvplot.xarray # noqa
hvplot.extension('matplotlib')

ds = hvplot.sampledata.air_temperature('xarray').sel(time='2014-02-25 12:00')

p = ds.hvplot.contour(
x='lon',
y='lat',
z='air',
# c='air',
levels=list(range(240, 300, 5)),
cmap='plasma_r',
linewidth=1.2,
data_aspect=1,
width=700,
clabel='Air Temperature (K)',
title='Contour Plot (Matplotlib)',
)
p
```