Fix "create an element for a token" with duplicate attributes

[domenic]

Fixes #1636.

[domenic]

/cc @dominiccooney @kojiishi as this may be a good thing to add test coverage for (i.e. <x-foo attr="1" attr="2"> should only fire one attributeChangedCallback, with value 1). (Does Daava have a GitHub handle?)

[annevk]

I wonder if I'm missing something around duplicate attributes. Not even @Hixie's old algorithm in 3a88aa5 handled it.

[annevk]

I did miss something. @zcorpan pointed out that a sentence following https://html.spec.whatwg.org/#attribute-name-state handles it.

[annevk]

My recommendation is that we close this PR and invalidate the issue. We could perhaps inline the statement following the Attribute name state as @zcorpan suggested.

[domenic]

Works for me.