[dom] getElementsByTagName needs to be clearer on when the same object can or can't be returned (#90)

In particular, it looks to me like at call-time the `getElementsByTagName` call captures the HTML-ness of _root_'s node document at the time it's called.  So if that HTML-ness changes, a new object needs to be returned.

I put a testcase at http://web.mit.edu/bzbarsky/www/testcases/dom-queries/test_getElementsByTagName_1.html which tries to exercise some of the edge-cases here.

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https://github.com/whatwg/dom/issues/90