Support default values for attributes of `anySimpleType`

[st-sloth]

Support of default values was largely implemented in #32.

But for attribute declarations of anySimpleType with default attribute, the resulting mappings don't have defaultValue property.

It was suggested in #32 to just copy the string value.

This PR adds example attribute declaration in the test schema.

[highsource]

@st-sloth This should work now, please give it a try.

[st-sloth]

Yes, it works perfectly, thank you!