Ensuring that checking and unchecking methods of the Checkbox control…

… work. Fixing #61.

src/SKUI/checkbox.rb

@@ -12,7 +12,7 @@ class Checkbox < Control
 
     # @return [Boolean]
     # @since 1.0.0
-    prop_reader_bool( :checked, &TypeCheck::BOOLEAN )
+    prop_writer( :checked, &TypeCheck::BOOLEAN )
 
     # @since 1.0.0
     define_event( :change )
@@ -31,25 +31,25 @@ def initialize( label, checked = false )
     # @return [Boolean]
     # @since 1.0.0
     def check!
-      checked = true
+      self.checked = true
     end
 
     # @return [Boolean]
     # @since 1.0.0
     def checked?
-      checked = window.bridge.get_checkbox_state( ui_id )
+      self.checked = window.bridge.get_checkbox_state( ui_id )
     end
 
     # @return [Boolean]
     # @since 1.0.0
     def toggle!
-      checked = !checked?
+      self.checked = !checked?
     end
 
     # @return [Boolean]
     # @since 1.0.0
     def uncheck!
-      checked = false
+      self.checked = false
     end
 
   end # class