Fallback to default attribute values when invalid values are used.

When an `ATTRS` setter returns `Y.Attribute.INVALID_VALUE` during
initialization, and a default attribute value is defined, fallback to
that value.

Fixes #2528732
Add more tests for Attribute default value fallback.
Merge branch 'attr-init-val' into dev-master

Conflicts:
	src/attribute/HISTORY.md
Build Attribute.