FIX: remove equality check in line2D.set_color

Fixes problem identified in matplotlib#4771

Author: tacaswell

lib/matplotlib/lines.py

@@ -932,9 +932,8 @@ def set_color(self, color):
 
         ACCEPTS: any matplotlib color
         """
-        if color != self._color:
-            self.stale = True
         self._color = color
+        self.stale = True
 
     def set_drawstyle(self, drawstyle):
         """