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

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: ```ruby 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.