Diffing_Engine: differences in strings are reported as differences in collections, not values

#### Description:

When a difference in string property is found, it is reported as `PropertyDifference` with `Description` formatted as _'The collection stored in the property (...) was modified'_, which is far less meaningful than a message about string changing from value A to B.

#### Test file(s):

On [SharePoint](https://burohappold.sharepoint.com/sites/BHoM/02_Current/Forms/AllItems.aspx?FolderCTID=0x0120008122C8891F89054B8ACED0196C70DFC4&id=%2Fsites%2FBHoM%2F02%5FCurrent%2F12%5FScripts%2F01%5FIssue%2FBHoM%2FBHoM%5FEngine%2FDiffing%5FEngine%2F%231492%2DStringDiffingBug)