Fix return values of String#casecmp

[znz]

see https://bugs.ruby-lang.org/issues/12835

調べられる範囲で調べてみたところ、1.8.6 でも TypeError でした。
String#+ などでも StringValue による @raise TypeError はいちいち書いていないので、返り値だけ変更しています。
-1, 0, 1 以外を返すことはなさそうだったので、Integer も Symbol#casecmp と同じく -1 | 0 | 1 に変更しました。

% rbenv each ruby -ve '"a".casecmp 1'
ruby 1.8.6 (2007-03-13 patchlevel 0) [x86_64-linux]
-e:1:in `casecmp': can't convert Fixnum into String (TypeError)
        from -e:1
ruby 1.8.6 (2010-09-02 patchlevel 420) [x86_64-linux]
-e:1:in `casecmp': can't convert Fixnum into String (TypeError)
        from -e:1
ruby 1.8.7 (2008-05-31 patchlevel 0) [x86_64-linux]
-e:1:in `casecmp': can't convert Fixnum into String (TypeError)
        from -e:1
ruby 1.8.7 (2013-12-22 patchlevel 375) [x86_64-linux]
-e:1:in `casecmp': can't convert Fixnum into String (TypeError)
        from -e:1
ruby 1.9.0 (2007-12-25 revision 14709) [x86_64-linux]
-e:1:in `casecmp': can't convert Fixnum into String (TypeError)
        from -e:1:in `<main>'
ruby 1.9.0 (2008-10-04 revision 19669) [x86_64-linux]
-e:1:in `casecmp': can't convert Fixnum into String (TypeError)
        from -e:1:in `<main>'
ruby 1.9.1p0 (2009-01-30 revision 21907) [x86_64-linux]
-e:1:in `casecmp': can't convert Fixnum into String (TypeError)
        from -e:1:in `<main>'
ruby 1.9.1p431 (2011-02-18 revision 30908) [x86_64-linux]
-e:1:in `casecmp': can't convert Fixnum into String (TypeError)
        from -e:1:in `<main>'
ruby 1.9.2p0 (2010-08-18 revision 29036) [x86_64-linux]
-e:1:in `casecmp': can't convert Fixnum into String (TypeError)
        from -e:1:in `<main>'
ruby 1.9.2p330 (2014-08-07 revision 47094) [x86_64-linux]
-e:1:in `casecmp': can't convert Fixnum into String (TypeError)
        from -e:1:in `<main>'
ruby 1.9.3p0 (2011-10-30 revision 33570) [x86_64-linux]
-e:1:in `casecmp': can't convert Fixnum into String (TypeError)
        from -e:1:in `<main>'
ruby 1.9.3p551 (2014-11-13 revision 48407) [x86_64-linux]
-e:1:in `casecmp': can't convert Fixnum into String (TypeError)
        from -e:1:in `<main>'
ruby 2.0.0p0 (2013-02-24 revision 39474) [x86_64-linux]
-e:1:in `casecmp': no implicit conversion of Fixnum into String (TypeError)
        from -e:1:in `<main>'
ruby 2.0.0p648 (2015-12-16 revision 53162) [x86_64-linux]
-e:1:in `casecmp': no implicit conversion of Fixnum into String (TypeError)
        from -e:1:in `<main>'
ruby 2.1.0p0 (2013-12-25 revision 44422) [x86_64-linux]
-e:1:in `casecmp': no implicit conversion of Fixnum into String (TypeError)
        from -e:1:in `<main>'
ruby 2.1.10p492 (2016-04-01 revision 54464) [x86_64-linux]
-e:1:in `casecmp': no implicit conversion of Fixnum into String (TypeError)
        from -e:1:in `<main>'
ruby 2.2.0p0 (2014-12-25 revision 49005) [x86_64-linux]
-e:1:in `casecmp': no implicit conversion of Fixnum into String (TypeError)
        from -e:1:in `<main>'
ruby 2.2.5p319 (2016-04-26 revision 54774) [x86_64-linux]
-e:1:in `casecmp': no implicit conversion of Fixnum into String (TypeError)
        from -e:1:in `<main>'
ruby 2.3.0p0 (2015-12-25 revision 53290) [x86_64-linux]
-e:1:in `casecmp': no implicit conversion of Fixnum into String (TypeError)
        from -e:1:in `<main>'
ruby 2.3.1p112 (2016-04-26 revision 54768) [x86_64-linux]
-e:1:in `casecmp': no implicit conversion of Fixnum into String (TypeError)
        from -e:1:in `<main>'

[stomar]

Note that String#casecmp can return nil:

"\u{e4 f6 fc}".encode("ISO-8859-1").casecmp("\u{c4 d6 dc}")   #=> nil

[znz]

#387 includes nil again.