Fix: Allow \r in unquoted fields when row separator doesn't contain \r

[jsxs0]

Fixes #60

This has been bugging me for a while - the CSV parser was rejecting \r characters in unquoted fields even when the row separator was something completely different like \n or a custom separator.

For example, this would fail unnecessarily:

CSV.parse("field1,field\rwith\rcr,field3\n", row_sep: "\n")

The problem was in prepare_unquoted where we were hardcoding "\r\n" instead of checking what the actual row separator was.

What changed:

• Now we only exclude characters that are actually part of the row separator
• If your row separator is \n, then \r is allowed in unquoted fields
• If your row separator is \r\n, then both \r and \n are still properly excluded
• Quoted fields work exactly the same as before

Testing:

• Updated the tests that were expecting the old behavior
• Added comprehensive tests for different row separator scenarios
• All existing tests still pass

This makes the parser more flexible while keeping it safe for the cases where \r should actually be restricted.

[jsxs0]

@kou I've made all the changes asked.

[jsxs0]

@kou Thank you. I have:

1. Removed the duplicate test
2. Moved the unique tests to test_general.rb
3. Fixed the misnamed test
4. Deleted the separate test file

[kou]

Thanks.