Fix LocalNetworkGateway, add tests

[lamchester]

Fix problems with ServiceManagement New-AzureLocalNetworkGateway and Get-AzureLocalNetworkGateway

• Prevent a null dereference issue when listing local network gateways (calling Get-AzureLocalNetworkGateway without parameters)
• Serialize the address space properly with New-AzureLocalNetworkGateway

Add tests/session records to prevent the first regression

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