CSV.parse can't handle double double-quoted column at Ruby 2.6.0

[Watson1978]

At first, I don't know whether the following CSV is valid format.

a,""b""

Ruby 2.5's CSV library handles it well.
However, Ruby 2.6's shows different behavior.

Ruby 2.6

$ ruby -v t.rb
ruby 2.6.0p0 (2018-12-25 revision 66547) [x86_64-darwin18]
Traceback (most recent call last):
	5: from t.rb:7:in `<main>'
	4: from /Users/watson/.rbenv/versions/2.6.0/lib/ruby/2.6.0/csv.rb:683:in `parse'
	3: from /Users/watson/.rbenv/versions/2.6.0/lib/ruby/2.6.0/csv.rb:1180:in `read'
	2: from /Users/watson/.rbenv/versions/2.6.0/lib/ruby/2.6.0/csv.rb:1180:in `to_a'
	1: from /Users/watson/.rbenv/versions/2.6.0/lib/ruby/2.6.0/csv.rb:1171:in `each'
/Users/watson/.rbenv/versions/2.6.0/lib/ruby/2.6.0/csv/parser.rb:273:in `parse': Do not allow except col_sep_split_separator after quoted fields in line 1. (CSV::MalformedCSVError)

Ruby 2.5

$ ruby -v t.rb
ruby 2.5.3p105 (2018-10-18 revision 65156) [x86_64-darwin18]
[["a", "\"b\""]]

Test code

require 'csv'

csv =<<CSV
a,""b""
CSV

p CSV.parse(csv)

[kou]

It should be parsable.

[kou]

Ah, this isn't a valid CSV. It should be quoted:

a,"""b"""

[kou]

This is a bug of csv in Ruby 2.5.

[kou]

But I don't like backward incompatibility.
I'll consider how to handle this.

[kou]

I've added liberal_parsing: {double_quote_outside_quote: true} option. The following code works on both Ruby 2.5 and Ruby 2.6 + csv master:

require 'csv'

csv =<<CSV
a,""b""
CSV

p CSV.parse(csv, liberal_parsing: {double_quote_outside_quote: true})