Fix `EncodingTransformation` when passing a `string` wrapped in `PSObject`

[santisq]

When a string is wrapped in PSObject the Encoding Transformation string => Encoding fails:

$utf8 = Write-Output utf8
Get-ZipEntry .\test.zip | Get-ZipEntryContent -Encoding $utf8

# Get-ZipEntryContent: Cannot process argument transformation on parameter 'Encoding'.
#                      Could not convert input 'utf8' to a valid Encoding object.