fix: accept a comma-delimited list in req.acceptsCharsets

[marceli1404]

Bug

req.acceptsCharsets() silently ignores the documented comma-delimited form. The jsdoc (added by merged PR #6088) documents:

req.acceptsCharsets('utf-8, utf-16');
// => utf-8

...but the implementation passes the arguments straight through to accepts().charsets(...), which never splits commas. So a client following the documented API gets a hard false:

// with request header `Accept-Charset: iso-8859-1, utf-8`
req.acceptsCharsets('iso-8859-1, utf-8');   // => false   (bug; iso-8859-1 IS acceptable)
req.acceptsCharsets('iso-8859-1', 'utf-8'); // => iso-8859-1  (multi-arg works)

Fix

Expand comma-delimited args before passing them along, so req.acceptsCharsets('iso-8859-1, utf-8') behaves like the documented multi-arg form:

req.acceptsCharsets = function(...charsets) {
  const accept = accepts(this);
  return accept.charsets(...charsets.flatMap((charset) =>
    Array.isArray(charset) ? charset : String(charset).split(',').map((s) => s.trim())
  ));
};

Verification

• New regression tests fail on the old code and pass on this change.
• Existing behavior unchanged: no-arg (returns the acceptable list), single-arg, multi-arg, and array inputs are byte-identical.
• npm test (mocha, full suite): 1262 passing, 0 failing.
• eslint clean on the changed files.

[krzysdz]

As far as I can tell, the docs are wrong. Comma delimited string has not been supported since cec0c06, according to #6936 (comment). You'll find more details and an attempt at fixing the docs in #7452.