Change Repository::save to accept an Item instead of an ItemDocument

[JaZo]

Description

See title.

Motivation and context

This change is based on feedback on the last (0.19.0) release. It removes the need to wrap the Item in an ItemDocument before giving it to the Repository. In 99% of the times you simply wrap the Item in an ItemDocument and directly pass it to the Repository. So the Repository can just as well do it for you making the implementing code cleaner.

How has this been tested?

Tested using updated unit tests.

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

Go over all the following points, and put an x in all the boxes that apply.

• I have read the CONTRIBUTING document.
• My pull request addresses exactly one patch/feature.
• I have created a branch for this patch/feature.
• Each individual commit in the pull request is meaningful.
• I have added tests to cover my changes.
• If my change requires a change to the documentation, I have updated it accordingly.