Expect attribute to be absent

[papandreou]

For some reason it was never possible to assert that an attribute is absent by going:

expect(element, 'to have attributes', { foo: undefined });
// or:
expect(element, 'to satisfy', { attributes: { foo: undefined } });

Analogous to the object to satisfy semantics.

This PR fixes that and adds tests.

[Munter]

Yay!