import private DNS zone file ignores serial in SOA

I'm importing DNS zone files with the command
`az network private-dns zone import -g $resourceGroup -n $dnsZoneName -f $file.FullName`

Everything works fine. Except the serial number I have in the SOA record is not importet or rather changed.
Even in the Portal (portal.azure.com) changes to the serial of SOA of a private DNS aren't working.

The same import command for public DNS (without the "private-") works well.
`az network dns zone import -g $resourceGroup -n $dnsZoneName -f $file.FullName`

Thank you for fixing

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#### Document Details

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* ID: 58d7dc28-2037-727e-35ff-84fe12c99810
* Version Independent ID: 24514e82-0693-773b-db77-ece9fef61693
* Content: [az network private-dns zone](https://docs.microsoft.com/en-us/cli/azure/network/private-dns/zone?view=azure-cli-latest#az-network-private-dns-zone-import)
* Content Source: [src/azure-cli/azure/cli/command_modules/network/_help.py](https://github.com/Azure/azure-cli/blob/dev/src/azure-cli/azure/cli/command_modules/network/_help.py)
* Service: **virtual-network**
* GitHub Login: @rloutlaw
* Microsoft Alias: **routlaw**