Restore proper pattern for attributes in elements.

It is important that test/test-element-attributes-markdown.html do test
the ability for the user to define its own pattern.
Restore default notation for attributes in element.

It should be `{. xxxx=yyy }`, not `{_ xxxx=yyy }`.
Only test/test-element-attributes-markdown.html were using `{_`,
simply in order to test the ability, for the user, to define a _different_ pattern.