When the info string already starts with `language-`, do not add the `language-` prefix

Should a change be made to the spec, so the behavior of cmark & commonmark.js matches the spec?

For context, see commonmark.js issue #277

Here’s what the spec currently says:

Although this spec doesn’t mandate any particular treatment of the info string, the first word is typically used to specify the language of the code block. In HTML output, the language is normally indicated by adding a class to the code element consisting of language- followed by the language name.