add form property to checkbox

src/mechanize/form.cr

@@ -61,7 +61,7 @@ class MechanizeCr::Form
       type = (html_node["type"] || "text").downcase
       case type
       when "checkbox"
-        @checkboxes << MechanizeCr::FormContent::CheckBox.new(html_node)
+        @checkboxes << MechanizeCr::FormContent::CheckBox.new(html_node, self)
       else
         @fields << MechanizeCr::FormContent::Field.new(html_node)
       end

src/mechanize/form/check_box.cr

@@ -1,9 +1,10 @@
 class MechanizeCr::FormContent::CheckBox < MechanizeCr::FormContent::Field
-  property :checked
+  property :checked, :form
 
-  def initialize(node : Node | Myhtml::Node, value = nil)
+  def initialize(node : Node | Myhtml::Node, form : MechanizeCr::Form)
     @checked = !!node["checked"]
-    super(node, value)
+    @form = form
+    super(node)
   end
 
   def check