Update-ModuleManifest corrupts PrivateData in module manifest

This bug by @jformacek is copied from https://github.com/MicrosoftDocs/PowerShell-Docs/issues/6767

Usage of this command on existing module manifest breaks it's content. I wanted to use this command inside build pipeline to update ModuleVersion attribute, but found at least 2 things that got broken on my manifest module:

• RequiredAssembles attribute: I have something like below in my manifest file, and Update-ModuleManifest removes this attribute completely

RequiredAssemblies = if($PSEdition -eq 'Desktop') {@('lib\net45\someAssembly.dll')} else {$null}

• PrivateData attribute: I have added additional content as hashtable Messages to module's PrivateData as addtional hashtable aside from PSData as shown below, however, Update-ModuleManifest changes the Messages hashtable to Messages = 'System.Collections.Hashtable', which breaks the logic of module

PrivateData = @{
    Messages = @{
        NotInitialized = 'Call Initialize-Module before running other cmdlets.'
    }
    PSData = @{
    # ... standard content
    }

Would be great to update the command so as it does not change the attributes that are not passed from command line, or at least document here what gets changed/reset when command is run, so as users are not surprised after running it.

Beginne mit dem Update-ModuleManifest-Befehl und reproduziere den Bericht mit einem Manifest, das RequiredAssemblies und verschachtelte PrivateData.Messages enthält. Vergleiche das Manifest vor und nach der Aktualisierung ausschließlich von ModuleVersion. Fertig ist die Aufgabe, wenn nicht übergebene Attribute intakt bleiben, einschließlich der verschachtelten Messages-Hashtable, mit Regressionstests für beide gemeldeten Fälle.