Make bibref query case-insensitive

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This addresses tobie/specref#22
Because the bibref test in https://github.com/lanthaler/specref/blob/f1b4e8940d2bd325d9a7f564842df85deb7774d6/test/bibref.js#L101 tests that the ref is returned as it was passed. People might enter things such as hTmL as references and that obviously wouldn’t be returned otherwise. I thought that’s the cleanest solution to not break anything that might still be case-sensitive.