Make bibref query case-insensitive

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This addresses tobie/specref#22
The explanation was in the commit messages of 2f5c235076246635cc7388e8364a13c5a9c181c7 and 0687e7f27c91b6a9d4d8f56d5ebc4c7a9bfc0d72. Let me briefly summarize it based on "html-20121217": There were both “html” and “HTML” in biblio.json. The former pointed to HTML5 at W3C and the latter and WHATWG’s living standard. This was highly confusing so I removed “html” which means that any form of html now always points to the living standard. There are no timestamped versions of the living standard so the ref “html-20121217” doesn’t exist anymore. We could manually introduce an alias for that, but I think it was never used anyway so I decided to simply remove these few cases from ref-list.json to pass the tests again.
In the unlikely case that someone needs this, we will find out soon and the fix is as simple as introducing a new alias. Sounds reasonable?