fix: add filename parameters to inline Content-Disposition headers

[michalsn]

Description
This PR fixes a bug in DownloadResponse::inline() method where filename parameters were missing for content disposition. This caused browsers to fall back to using the last URL segment as the filename instead of the intended filename.

The browser will display the filename that was set for image and PDF files.

I added addslashes() because if the filename contains a quote character ("), it would break the header syntax by prematurely terminating the quoted string.

The problem was recognized here: https://forum.codeigniter.com/showthread.php?tid=93147

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[samsonasik]

Thank you @michalsn