plot_date: Set the default fmt to 'o'

Different to just plot() plot_date() has a default plot format string
which makes it necessary to *unset* the default one when drawing
multiple sequences of dates in a figure to get a working color cycling.
It also was not exactly clear that Axes.set_color_cycle was setting the
color cycle for every plot command except for plot_date ones.

It was of course possible to simply unset it after reading the
documentation but that's not the path of least suprise.

The default fmt of 'o' will keep the markers by default (which makes
sense for dates) but allows color cycling to happen.

This is related to issue #2148

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I'm the one who asked the question on SO that's linked to in issue #2148 and I just had to give this a try with a pull request because I think the discussion on the ticket was really short. Imho, setting the default format to just 'o' which keeps the markers but allows color cycling to happen is a reasonable compromise.