Match String case folding in StrBuilder.equalsIgnoreCase

[alhudz]

Repro: new StrBuilder("K").equalsIgnoreCase(new StrBuilder("K")) (KELVIN SIGN vs K) returns false, and so does the å/ANGSTROM pair new StrBuilder("å").equalsIgnoreCase(new StrBuilder("Å")). String.equalsIgnoreCase returns true for both.

Cause: the per-character loop only rules a differing pair equal when Character.toUpperCase(c1) == Character.toUpperCase(c2). Characters that fold together only through their lower-case mapping (U+212A → k, U+212B → å) fail that test, so the whole compare reports not-equal.

Fix: also accept the pair when Character.toLowerCase of the upper-cased chars agrees, the same fold String#regionMatches and this library's own CharSequenceUtils.equalsIgnoreCase already use. The case-sensitive equals sibling is untouched.