Changes in 1.5.3

``ax.plot(..., marker=None)`` gives default marker

Prior to 1.5.3 kwargs passed to `~matplotlib.Axes.plot` were handled

in two parts -- default kwargs generated internal to

`~matplotlib.Axes.plot` (such as the cycled styles) and user supplied

kwargs. The internally generated kwargs were passed to the

`matplotlib.lines.Line2D.__init__` and the user kwargs were passed to

``ln.set(**kwargs)`` to update the artist after it was created. Now

both sets of kwargs are merged and passed to

`~matplotlib.lines.Line2D.__init__`. This change was made to allow `None`

to be passed in via the user kwargs to mean 'do the default thing' as

is the convention through out mpl rather than raising an exception.

Unlike most `~matplotlib.lines.Line2D` setter methods

`~matplotlib.lines.Line2D.set_marker` did accept `None` as a valid

input which was mapped to 'no marker'. Thus, by routing this

``marker=None`` through ``__init__`` rather than ``set(...)`` the meaning

of ``ax.plot(..., marker=None)`` changed from 'no markers' to 'default markers

from rcparams'.

This is change is only evident if ``mpl.rcParams['lines.marker']`` has a value

other than ``'None'`` (which is string ``'None'`` which means 'no marker').