Match String case folding in TextStringBuilder.equalsIgnoreCase

[alhudz]

Ports apache/commons-lang#1745 to TextStringBuilder, as requested there.

Repro: new TextStringBuilder("K").equalsIgnoreCase(new TextStringBuilder("K")) (KELVIN SIGN U+212A vs K) returns false, and so does the å/ANGSTROM pair new TextStringBuilder("å").equalsIgnoreCase(new TextStringBuilder("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 uses. Regression assertions added to TextStringBuilderTest#testEqualsIgnoreCase fail at the Kelvin pair without the main change and pass with it; all 124 tests in the class pass, and the default mvn build is green.

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