#247 Mixed content is not inherited

[Michael Kay]

In XML Schema, when type A is derived from type B by
extension, and type B specifies mixed="true", and type
B does not specify mixed, and type A's content model is
empty, then type A should allow mixed content (in fact,
it should allow exactly the same content as B). But
Saxon treats it as if it specified mixed="false", and
reports this as a schema validity error.

Not yet fixed.