setRelation does the same thing as setAttribute

[domenic]

This might be related to #21 and #22. But both of them store the value in the source, which seems like it would cause collisions.

[minorninth]

Except the valid attributes and valid relations are different?

I think there's utility in saving unrecognized values, but not unrecognized attribute names.

What I intended the spec to imply is that setAttribute('role', 'santa') would be saved in the source, but not used because it's not valid, whereas setAttribute('z-index', 55) would throw an exception or do nothing because 'z-index' is not even a recognized attribute name.