StringToEnumConverterFactory class from enum value

Update StringToEnumConverterFactory to search superclasses until
Class.isEnum() returns true. This allows conversion when the
enum class is obtained from the enum value:

    public static enum SubFoo {
        BAR { String s() { return "x"; } };
        abstract String s();
    }

    conversionService.convert("BAR", SubFoo.BAR.getClass())

This fix is particularly important when converting collections of
enums.

Issue: SPR-10329