fix: Ensure Content-Disposition Header conforms to RFC6266

[sh41]

What kind of change does this PR introduce?

Bug fix.

What is the current behavior?

When a filename is specified, the content-disposition header is added. Currently the header will have content like this:

attachment; filename=file.txt; filename*=UTF-8''file.txt;

RFC6266 specifies the valid formats for this header, and a trailing semi-colon is not allowed per the grammar at https://datatracker.ietf.org/doc/html/rfc6266#page-4

I was prompted to raise this PR when I ran into the same issue discussed here: jshttp/content-disposition#11

What is the new behavior?

The PR removes the trailing semi-colon and updates the test for it.

attachment; filename=file.txt; filename*=UTF-8''file.txt

[fenos]

Thanks a lot, @sh41, good catch!