BUGFIX: merged patch from ajshort to fix checkbox fields in the cms

git-svn-id: svn://svn.silverstripe.com/silverstripe/open/modules/sapphire/trunk@64770 467b73ca-7a2a-4603-9d3b-597d59a354a9

forms/CheckboxField.php

@@ -19,7 +19,7 @@ function Field() {
 			'class' => ($this->extraClass() ? $this->extraClass() : ''),
 			'id' => $this->id(),
 			'name' => $this->Name(),
-			'value' => 1,
+			'checked' => $this->value ? 'checked' : '',
 			'tabindex' => $this->getTabIndex()
 		);