Currently, `Integral#chr` in mruby changes behavior by `MRB_UTF8_STRING`
setting.
before this patch:
$ bin/mruby -e 'p 171.chr' #=> "\xab" (`MRB_UTF8_STRING` is disabled)
$ bin/mruby -e 'p 171.chr' #=> "«" (`MRB_UTF8_STRING` is enabled)
This behavior is incompatible with Ruby, and a little inconvenient because
it can't be interpreted as ASCII-8BIT with `MRB_UTF8_STRING`, I think.
So add encoding argument according to Ruby.
after this patch:
$ bin/mruby -e 'p 171.chr' #=> "\xab"
$ bin/mruby -e 'p 171.chr("ASCII-8BIT")' #=> "\xab"
$ bin/mruby -e 'p 171.chr("UTF-8")' #=> "«"
Allow only `String` for encoding because mruby doesn't have `Encoding`
class, and `"ASCII-8BIT"` (`"BINARY"`) and `"UTF-8"` (only with
`MRB_UTF8_STRING`) are valid value (default is `"ASCII-8BIT"`).
Because they're defined in both `mruby-string-ext` and `mruby-numeric-ext`
(they seem more natural to define in N, but `mruby-string-ext` depends on
`Integral#chr`).
John Bampton