Custom header: Avoid warnings in the process_default_headers() method.

props obenland.
fixes #27850.

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Multisite: Fix case sensitivity regressions in 3.9.

props jeremyfelt.
fixes #27866.

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In `WP_List_Table::display_rows_or_placeholder()`, the call to `get_column_info()` in unnecessary and unused. The call to `get_column_count()` immediately after makes the same call internally.

See #27882.


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Multisite: Treat 'www' as a special subdomain, reversing 3.9 regression.

props jeremyfelt.
fixes #27927.

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Where a filter no longer exists, it need not be notated.

Fixes #24444. See r28258.

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