Re: [whatwg/dom] Make the DOMTokenList creation steps use "get an attribute value" (#673)

Sorry, I was reshuffling to not base my pulls off my fork's master branch, so I had to recreate this pull request.

Just a few thoughts.

#### Current behavior
* Uses "get an attribute by namespace and local name", which returns an `Attr` node or `null`
* This only worked if the element did not have an attribute that was the associated attribute's local name and therefore, "get an attribute by namespace and local name" would return null.
* If the value is null, the "attribute change steps" would use "Step 1" and empty the *token set*.
* If the value is an `Attr` node, the "attribute change steps" will use "Step 2" and will try to run the "ordered set parsing" steps on the `Attr` node. (I guess the best case scenario here is that it gets serialized to "[object Attr]" and the "ordered set parsing" steps return a new *token set* containing "[object" and "Attr]")

#### New behavior
* Uses "get an attribute value", which always returns a string.
* The "attribute change steps" will now always use "Step 2", which runs the string through the "ordered set parsing" steps. (This now always replaces the *token set* with a new *token set* instead of just emptying the existing *token set* when an attribute with the associated local name is not present, since we always pass a string unlike the current behavior, which passes null when the attribute is not present.)

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