How to exclude the submit action from a list of parameters in Struts 2?

I'm trying to exclude a submit action from a parameter list.

The following is the action class:

@Namespace("/admin_side")
@ResultPath("/WEB-INF/content")
@InterceptorRefs({
    @InterceptorRef(value="validation", params={"excludeMethods", "test"}),
    @InterceptorRef(value="params", params={"excludeParams", "action:postAction"})})
public final class TestAction extends ActionSupport implements Serializable, ValidationAware
{
    private static final long serialVersionUID = 1L;
    private static final String SUCCESS = "success";

    private String name;

    @Action(value = "test", results = {
    @Result(name="success", location="Test.jsp"),
    @Result(name = "input", location = "Test.jsp")})
    public String test() throws Exception
    {
        System.out.println("name = "+name);
        return SUCCESS;
    }

    @Action(value = "postAction", results = {
    @Result(name="success", location="Test.jsp"),
    @Result(name = "input", location = "Test.jsp")})
    public String postAction()
    {
        System.out.println("Post invoked");
        System.out.println("name = "+name);
        return SUCCESS;
    }

    @RequiredStringValidator(type= ValidatorType.FIELD, message = "The name is required.")
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public void validate()
    {
        if(!hasErrors()&&name.length()<2)
        {
            addFieldError("name", "The name must compose of 2 letters.");
        }
    }
}

The Test.jsp page:

<s:form namespace="/admin_side" action="test" validate="true">
    <s:textfield id="name" name="name" label="Name"/>
    <s:submit id="btnSubmit" name="btnSubmit" value="Submit" action="postAction"/>
</s:form>

The generated HTML code for <s:submit> would be like as follows.

<input type="submit" id="btnSubmit" name="action:postAction" value="Submit"/>

The @InterceptorRef above the class:

@InterceptorRef(value="params", params={"excludeParams", "action:postAction"})

doesn't seem to work. The method postAction() is never invoked causing the following warning to be issued.

Dec 24, 2013 10:49:16 PM com.opensymphony.xwork2.interceptor.ParametersInterceptor warn WARNING: Parameter [action:postAction] is on the excludeParams list of patterns!

In the struts.properties file, I have the following properties until now.

struts.enable.DynamicMethodInvocation=false
struts.devMode=false
struts.ui.theme=simple

struts.convention.package.locators=actions
struts.convention.action.suffix=Controller
struts.convention.action.mapAllMatches=true

struts.convention.result.path=/WEB-INF/content //No need. It is the default.
struts.mapper.action.prefix.enabled=true

I'm using Struts 2.3.16.

Why doesn't it exclude the submit button's parameter? How to invoke the postAction() method, when <s:submit> is clicked?