`cmap` option ignored in a line plot

Extracted from https://github.com/holoviz/hvplot/issues/652 where the `cmap` option neither has the expected effect nor raises a warning/error:

```python
import holoviews as hv
import hvplot.pandas
import numpy as np
import pandas as pd

df = pd.DataFrame({
'x': list(np.arange(10)) * 6,
'y': np.concatenate([np.arange(10) + x for x in range(6)]),
'z': [l for l in range(0, 60, 10) for _ in range(10)],
})

df.hvplot.line('x', 'y', by='z', cmap='Reds')
```