FIX: treat zorder=None as falling back to the default

closes matplotlib#9785

Author: tacaswell

lib/matplotlib/artist.py

@@ -885,6 +885,8 @@ def set_zorder(self, level):
 
         ACCEPTS: any number
         """
+        if level is None:
+            level = self.__class__.zorder
         self.zorder = level
         self.pchanged()
         self.stale = True

lib/matplotlib/tests/test_artist.py

@@ -246,3 +246,13 @@ def test_setp():
     sio = io.StringIO()
     plt.setp(lines1, 'zorder', file=sio)
     assert sio.getvalue() == '  zorder: any number \n'
+
+
+def test_None_zorder():
+    fig, ax = plt.subplots()
+    ln, = ax.plot(range(5), zorder=None)
+    assert ln.get_zorder() == mlines.Line2D.zorder
+    ln.set_zorder(123456)
+    assert ln.get_zorder() == 123456
+    ln.set_zorder(None)
+    assert ln.get_zorder() == mlines.Line2D.zorder