Fix widget.disabled handling of value change of equal truthiness

Comparing for equality is too strict, when what we want is to know if
thue value is Truthy or Falsy, this mismatch in condition created cases
where a widget was set to disabled, but wasn't, as well as widgets
staying disabled, when they should have been enabled again.

Author: tshirtman

kivy/tests/test_widget.py

@@ -118,3 +118,10 @@ def test_export_to_png(self):
         rmtree(tmp)
 
         self.root.remove_widget(wid)
+
+    def test_disabled(self):
+        from kivy.uix.widget import Widget
+        w = Widget(disabled=None)
+        w.disabled = False
+        w.disabled = True
+        self.assertEqual(w.disabled, True)

kivy/uix/widget.py

@@ -1433,6 +1433,9 @@ def get_disabled(self):
         return self._disabled_count > 0
 
     def set_disabled(self, value):
+        # Necessary to ensure a change between value of equal truthiness
+        # doesn't mess up the count
+        value = bool(value)
         if value != self._disabled_value:
             self._disabled_value = value
             if value: