Panic if DOMTokenList#contains is called for an unparsed attribute.

Previously, if the attribute was not parsed into a token list, and the
tokens() method returned None, DOMTokenList#contains would silently return
false. This issue was encountered in
<https://github.com/servo/servo/pull/4076> and took quite some time to
figure out.
auto merge of #4084 : Ms2ger/servo/tokenlist-robust, r=jdm

Previously, if the attribute was not parsed into a token list, and the
tokens() method returned None, DOMTokenList#contains would silently return
false. This issue was encountered in
<https://github.com/servo/servo/pull/4076> and took quite some time to
figure out.