Revert "Rollback to released version numbers of stringio and strscan"
This reverts commit 6a79e53823e328281b9e9eee53cd141af28f8548.
[ruby/strscan] StringScanner#captures: Return nil not "" for unmached capture (https://github.com/ruby/strscan/pull/72)
fix https://github.com/ruby/strscan/issues/70
If there is no substring matching the group (s[3]), the behavior is
different.
If there is no substring matching the group, the corresponding element
(s[3]) should be nil.
```
s = StringScanner.new('foobarbaz') #=> #<StringScanner 0/9 @ "fooba...">
s.scan /(foo)(bar)(BAZ)?/ #=> "foobar"
s[0] #=> "foobar"
s[1] #=> "foo"
s[2] #=> "bar"
s[3] #=> nil
s.captures #=> ["foo", "bar", ""]
s.captures.compact #=> ["foo", "bar", ""]
```
```
s = StringScanner.new('foobarbaz') #=> #<StringScanner 0/9 @ "fooba...">
s.scan /(foo)(bar)(BAZ)?/ #=> "foobar"
s[0] #=> "foobar"
s[1] #=> "foo"
s[2] #=> "bar"
s[3] #=> nil
s.captures #=> ["foo", "bar", nil]
s.captures.compact #=> ["foo", "bar"]
```
https://docs.ruby-lang.org/ja/latest/method/MatchData/i/captures.html
```
/(foo)(bar)(BAZ)?/ =~ "foobarbaz" #=> 0
$~.to_a #=> ["foobar", "foo", "bar", nil]
$~.captures #=> ["foo", "bar", nil]
$~.captures.compact #=> ["foo", "bar"]
```
* StringScanner#captures is not yet documented.
https://docs.ruby-lang.org/ja/latest/class/StringScanner.html
https://github.com/ruby/strscan/commit/1fbfdd3c6f
[ruby/strscan] Bump version
https://github.com/ruby/strscan/commit/d6f97ec102