[ruby/csv] CSV.generate_line: use the encoding of the first non ASCII field as the expected encoding

See also: https://github.com/ruby/stringio/issues/13#issuecomment-660543554

https://github.com/ruby/csv/commit/004cf49d18

lib/csv.rb

@@ -1289,8 +1289,20 @@ class CSV
       str = +""
       if options[:encoding]
         str.force_encoding(options[:encoding])
-      elsif field = row.find {|f| f.is_a?(String)}
-        str.force_encoding(field.encoding)
+      else
+        fallback_encoding = nil
+        output_encoding = nil
+        row.each do |field|
+          next unless field.is_a?(String)
+          fallback_encoding ||= field.encoding
+          next if field.ascii_only?
+          output_encoding = field.encoding
+          break
+        end
+        output_encoding ||= fallback_encoding
+        if output_encoding
+          str.force_encoding(output_encoding)
+        end
       end
       (new(str, **options) << row).string
     end

test/csv/test_encodings.rb

@@ -242,6 +242,18 @@ class TestCSVEncodings < Test::Unit::TestCase
     assert_equal("UTF-8",      data.to_csv.encoding.name)
   end
 
+  def test_encoding_is_not_upgraded_for_non_ascii_content_during_writing_as_needed
+    data = ["\u00c0".encode("ISO-8859-1"), "\u3042"]
+    assert_equal([
+                   "ISO-8859-1",
+                   "UTF-8",
+                 ],
+                 data.collect {|field| field.encoding.name})
+    assert_raise(Encoding::CompatibilityError) do
+      data.to_csv
+    end
+  end
+
   def test_explicit_encoding
     bug9766 = '[ruby-core:62113] [Bug #9766]'
     s = CSV.generate(encoding: "Windows-31J") do |csv|