Issue #9766 has been reported by DAISUKE TANIWAKI.

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Feature #9766: Add force_encoding option to csv
https://bugs.ruby-lang.org/issues/9766

* Author: DAISUKE TANIWAKI
* Status: Open
* Priority: Normal
* Assignee: 
* Category: lib
* Target version: current: 2.2.0
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Hi there,

I have a trouble when I use csv#generate with encoding 'Shift-JIS' option. I investigated it for a long time and found it is caused by compatibility within "UTF-8" and "Shift-JIS". Since "Shift-JIS" can be converted to UTF-8,a row with UTF-8 strings added to the csv instance makes the encoding of whole rows UTF-8.

Take a look at the code below.
https://github.com/dtaniwaki/ruby/blob/trunk/lib/csv.rb#L1658

Here's the code example.

```rb
irb(main):002:0> s = generate(encoding: 'SJIS') do |csv|
    csv << ['あ']
  end
=> ["あ"]

irb(main):003:0> s
=> "あ\n"

irb(main):004:0> s.encoding
=> #<Encoding:UTF-8>
```

I was intended to make SJIS encoded csv, but the result was UTF-8 csv. I think everyone think it should generate Shift-JIS encoded csv string, so could you consider to merge the change attached to this issue?

The expected result is here.

```rb
irb(main):002:0> s = generate(encoding: 'SJIS', force_encoding: true) do |csv|
    csv << ['あ']
  end
=> ["あ"]

irb(main):003:0> s
=> "\x{E381}\x82\n"

irb(main):004:0> s.encoding
=> #<Encoding:Windows-31J>
```


---Files--------------------------------
csv.rb.diff (1.41 KB)


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