Fixes #5257

Problem

htmlparser2 stores HTML attribute names in lowercase (per the HTML spec), but removeAttr looked them up using the caller-supplied casing. As a result, removeAttr('CLASS') failed to remove a parsed class attribute:

const $ = cheerio.load('<div class="test"></div>');
$('div').removeAttr('CLASS');
$('div').attr('class'); // 'test'  — expected undefined

Solution

Lowercase the attribute name before lookup when not in xmlMode, mirroring how the DOM treats HTML attribute names as case-insensitive. This follows the existing xmlMode convention already used by getAttr/setProp in this file.

• HTML mode: removeAttr('CLASS') now removes a class attribute.
• XML mode: casing is preserved (unchanged), since XML attribute names are case-sensitive.

The shared removeAttribute helper (also used by setAttr for null-value deletes) is intentionally left untouched, so programmatic case-sensitive behavior via attr(name, null) is unaffected.

Tests

Added two cases to .removeAttr:

• HTML mode — uppercase name removes the lowercased attribute.
• XML mode — a differently-cased name does not remove the attribute; the exact name still does.

Full suite: 792 passed, no type errors, lint clean.