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.

Update-ModuleManifest コマンドから始め、RequiredAssemblies と入れ子になった PrivateData.Messages を含むマニフェストで報告を再現します。ModuleVersion だけを更新する前後でマニフェストを比較します。未指定の属性が、入れ子になった Messages ハッシュテーブルを含めてそのまま保持され、報告された両方のケースに対するリグレッションカバレッジがあることを完了条件とします。