[dom] Exception in DOMTokenList.add() and DOMTokenList.remove() algo (#93)

I don't see (test Firefox and Chrome) that any of this two kind of exception is more important, what actual algorithms suggest. Maybe change add() to:

1. For each token in tokens, in given order, run these substeps: 
1.1 If token is the empty string, throw a SyntaxError exception.
1.2 If token contains any ASCII whitespace, then throw an InvalidCharacterError exception.
1.3  If token is not in tokens, append token to tokens. 
2. Run the update steps. 

and similarly for remove().

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https://github.com/whatwg/dom/issues/93