Update-ModuleManifest complains when adding an invalid entry value, yet still updates the manifest.

[pougetat]

This is a regression from Windows PowerShell.

This has only been tried with one entry, RootModule, where the bug only surfaces if the input manifest file does not have the entry being updated yet.

Steps to reproduce

# Important: create the manifest *without* -RootModule
New-ModuleManifest tmp.psd1
# Try to update (add) the RootModule entry with an invalid value.
Update-ModuleManifest tmp.psd1 -RootModule '\/'
'---'
# Extract the now-current RootModule entry.
Select-String ^RootModule tmp.psd1

Expected behavior

Microsoft.PowerShell.Core\Test-ModuleManifest : The module manifest '\/' could not be processed because it is not a valid PowerShell module manifest file.
...
---

Actual behavior

Microsoft.PowerShell.Core\Test-ModuleManifest : The module manifest '\/' could not be processed because it is not a valid PowerShell module manifest file.
...
---

tmp.psd1:12:RootModule = '\/'

That is, the manifest file was unexpectedly still updated with the invalid entry.

Environment data

PowerShell Core 6.2.0-preview.1 on macOS 10.14.1
PowerShell Core 6.2.0-preview.1 on Ubuntu 16.04.5 LTS
PowerShell Core 6.2.0-preview.1 on Microsoft Windows 10 Pro (64-bit; Version 1803, OS Build: 17134.345)

[pougetat]

@SteveL-MSFT Hey is this issue up for grabs ? If it is I was wondering if I could work on it :)

[SydneyhSmith]

@pougetat we would love it if you worked on this issue! Feel free to assign it to yourself.

[alerickson]

Thanks @pougetat!