datetime handling for spikes

[poplarShift]

The previous implementation of get_data for Spikes in the matplotlib backend used the element.array() method, which already coerces datetime values to int in case of mixed data types. I fixed that and updated the datetime handling using recently added utils.

The following now works, whereas before it didn't:

import pandas as pd
import numpy as np
import holoviews as hv
hv.extension('matplotlib')

e = hv.Spikes((pd.date_range('2100-1-1', '2100-10-10', periods=20), np.random.rand(20)), 'Date', 'c')
e.opts(xrotation=45)

[philippjfr]

Thanks @poplarShift!

[jlstevens]

Yes, thank you @poplarShift!