Improve backwards compatibility of ErrorAttributes and DefaultErrorAttributes

[wilkinsona]

The changes for including the message or binding errors are perhaps more breaking than they need to be. I’ve been wondering if we should make the new method on ErrorAttributes a default method. I also wondered if we should encapsulate the three include… booleans into a type so that we can add more exclusions in the future without having to change that interface.

As an alternative to a new type that encapsulates the includes, @philwebb suggested a varargs enum. An implementation could then use EnumSet.copyOf(Arrays.asList(includes)).