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.

Start at the Update-ModuleManifest command and reproduce the report with a manifest containing RequiredAssemblies and nested PrivateData.Messages. Compare the manifest before and after updating only ModuleVersion. Done means unpassed attributes remain intact, including the nested Messages hashtable, with regression coverage for both reported cases.