MediaWiki writer: fix code for generating spans for header IDs.

We need to generate a span when the header's ID doesn't match
the one MediaWiki would generate automatically.  But MediaWiki's
generation scheme is different from ours (it uses uppercase letters,
and `_` instead of `-`, for example).

This means that in going from markdown -> mediawiki, we'll now get
spans before almost every heading, unless explicit identifiers are
used that correspond to the ones MediaWiki auto-generates.
This is uglier output but it's necessary for internal links to
work properly.

See #7697.

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