[share_plus] fix 'Save Image' option not showing

[Coronon]

Description

The 'Save Image' option was not showing when invoking the ShareSheet with share_plus after 4.0.6 which switched to use the before abandoned ShareDate class again. The implementation of activityViewControllerPlaceholderItem did not provide enough context to the OS however to display the image specific options. Other apps that allow sharing were not effected as they relied upon itemForActivityType.

As always, I provide a working example here.

Related Issues

Fixes #897

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated the version in pubspec.yaml and CHANGELOG.md.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[miquelbeltran]

thanks a lot @Coronon for taking care of this! I will release asap