Fix FreeMarker form checbox macro generated names

In Spring 3.2 a fix was implemented for all Spring Freemarker form
macros so that generated form input fields have valid bracketless IDs.
In the fix a regression was introduced manifesting in formCheckbox
macro no longer generating appropriate names for checkbox and hidden
input field.

This change fixes mentioned regression issue.

Issue: SPR-8732

spring-webmvc/src/main/java/org/springframework/web/servlet/view/freemarker/spring.ftl

@@ -309,8 +309,8 @@
 	<@bind path />
     <#assign id="${status.expression?replace('[','')?replace(']','')}">
     <#assign isSelected = status.value?? && status.value?string=="true">
-	<input type="hidden" name="_${id}" value="on"/>
-	<input type="checkbox" id="${id}" name="${id}"<#if isSelected> checked="checked"</#if> ${attributes}/>
+	<input type="hidden" name="_${status.expression}" value="on"/>
+	<input type="checkbox" id="${id}" name="${status.expression}"<#if isSelected> checked="checked"</#if> ${attributes}/>
 </#macro>
 
 <#--