Add encoding argument to `Integral#chr`

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"`).

1 files changed, 8 insertions, 1 deletions

diff --git a/mrbgems/mruby-string-ext/test/string.rb b/mrbgems/mruby-string-ext/test/string.rb
index 2eb35f840..8f1d25f29 100644
--- a/mrbgems/mruby-string-ext/test/string.rb
+++ b/mrbgems/mruby-string-ext/test/string.rb
@@ -167,8 +167,15 @@ end
 assert('String#concat') do
   assert_equal "Hello World!", "Hello " << "World" << 33
   assert_equal "Hello World!", "Hello ".concat("World").concat(33)
-
   assert_raise(TypeError) { "".concat(Object.new) }
+
+  if UTF8STRING
+    assert_equal "H«", "H" << 0xab
+    assert_equal "Hは", "H" << 12399
+  else
+    assert_equal "H\xab", "H" << 0xab
+    assert_raise(RangeError) { "H" << 12399 }
+  end
 end
 
 assert('String#casecmp') do