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"`).