Allow a default value to be returned when property is not found by BeanELResolver.

It would be better to handle it on some higher level, probably AST,  but I'm already happy with this for my purposes.

Usage:

    /**
     *  Puts a custom default string in place of unresolved property, instead of throwing an ex.
     */
    public static class BeanELDefaultStringResolver extends BeanELOpenResolver {

        private final String defaultString;

        public BeanELDefaultStringResolver( String defaultString ) {
            this.defaultString = defaultString;
        }

        @Override protected Object onPropertyNotFoundRead( Object base, Object property ) {
            return this.defaultString;
        }
    }