Fix saving work items when impersonation is enabled

[aok-foss]

Fixes #229

Updates WorkItemWrapper field assignment logic so that repeated assignments of the same fields result in correct values of Changes and IsDirty. In particular, setting a field to null and then to a non-null value will no longer result in emitting a Remove operation with a non-null value.

[giuliov]

I opted for a different solution when a field is reset to original value and dirty flag should be cleared.

You can see it in the release/1.1 branch.