Changed deafult filter name to a constant

I've just changed ""springSecurityFilterChain" String to a constant named <code>DEFAULT_FILTER_NAME</code>.

When I tried to register a <code>DelegatingFilterProxy</code> for the Spring Security's fileter chain bean, I couldn't remember the target bean name and after I found it I felt it is not easy to type(it is a little bit long to type ti exactly).

I think this will be helpful to those who want to use Spring Security JavaConfig.

now

```java
@Override
protected Filter[] getServletFilters() {
    ...
    DelegatingFilterProxy securityFilter = new DelegatingFilterProxy();
    securityFilter.setTargetBeanName("springSecurityFilterChain");

    ...
}
```

hope to

```java
@Override
protected Filter[] getServletFilters() {
    ...
    DelegatingFilterProxy securityFilter = new DelegatingFilterProxy();
    securityFilter.setTargetBeanName(WebSecurityConfiguration.DEFAULT_FILTER_NAME);

    ...
}
```

I'm not sure if the constant is on the right place. You can move it to wherever you want.

Thanks.