Fix problem with zip file creation on Windows

[t-hamano]

Fixes #602

This PR fixes an issue where files are not created correctly when using certain file archivers on Windows OS.

Where the problem occurs, the file is generated by ZipArchive::addFromString. The PHP documentation mentions the following:

Note: For maximum portability, it is recommended to always use forward slashes (/) as directory separator in ZIP filenames.

I don't know the exact cause, but I think backslashes are causing problems with certain file archivers.

Testing Instructions

• Check out this branch on Windows.
• Download & install 7-Zip.
• Run "Export Zip" in the Site Editor.
• Unzip the file using 7-zip.

Screenshot

Before	After

[nicholasingham]

Just a thought. The plugin code doesn't use the path_join function at the moment.

Syntax such as $a. '/' . $b or 'pattern/' . $c is used throughout.

For consistency, would it be better to continue to use the . '/' . syntax for this PR and decide whether to change to path_join everywhere later?

[vcanales]

Just a thought. The plugin code doesn't use the path_join function at the moment.

Syntax such as $a. '/' . $b or 'pattern/' . $c is used throughout.

For consistency, would it be better to continue to use the . '/' . syntax for this PR and decide whether to change to path_join everywhere later?

We're introducing it as a part of another PR as well #601.

There are also places where it might not be suitable to use it ie. where DIRECTORY_SEPARATOR is in use.

I don't see a drawback in introducing it here, and also gradually whenever we need to make changes to path handling logic.

[nicholasingham]

Makes sense. Many thanks.

[pbking]

Took longer to get my windows environment working again then it did to test it (and for just a 2 line change at that 🤣 )

Works as expected. The unpacked zip archive looks as expected.

Thanks for this fix.