Fix casefold

Author: msimacek

graalpython/com.oracle.graal.python.test/src/tests/unittest_tags/test_unicode.txt

@@ -15,6 +15,7 @@
 *graalpython.lib-python.3.test.test_unicode.UnicodeTest.test_capitalize
 *graalpython.lib-python.3.test.test_unicode.UnicodeTest.test_capitalize_nonascii
 *graalpython.lib-python.3.test.test_unicode.UnicodeTest.test_case_operation_overflow
+*graalpython.lib-python.3.test.test_unicode.UnicodeTest.test_casefold
 *graalpython.lib-python.3.test.test_unicode.UnicodeTest.test_center
 *graalpython.lib-python.3.test.test_unicode.UnicodeTest.test_codecs
 *graalpython.lib-python.3.test.test_unicode.UnicodeTest.test_codecs_idna

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/str/StringBuiltins.java

@@ -2269,9 +2269,7 @@ public abstract static class CasefoldNode extends PythonUnaryBuiltinNode {
         @Specialization
         @TruffleBoundary
         static String doString(String self) {
-            // TODO(fa) implement properly using 'unicodedata_db' (see 'unicodeobject.c' function
-            // 'unicode_casefold_impl')
-            return self.toLowerCase();
+            return UCharacter.foldCase(self, true);
         }
 
         @Specialization(replaces = "doString")