Cover all subnet properties during VNet update

[JunyiYi]

This PR fixes #1619 . To prevent future changes, it also covers all other properties by returning the response directly instead of copying the fields one-by-one.

[JunyiYi]

Tests (including all vnet related tests as well as subnet tests) passed:

[metacpp]

@JunyiYi Thanks for the fix and it LGTM.

[katbyte]

Thanks for the fix @JunyiYi, LGTM and tests pass:

OK	TestAccAzureRMSubnet_routeTableUpdate 	4m:06s,622ms 	14
OK	TestAccAzureRMSubnet_routeTableRemove 	3m:55s,018ms 	13
OK	TestAccAzureRMSubnet_basic 	3m:53s,808ms 	12
OK	TestAccAzureRMSubnet_serviceEndpointsVNetUpdate 	3m:53s,522ms 	11
OK	TestAccAzureRMSubnetNetworkSecurityGroupAssociation_deleted 	3m:52s,431ms 	10
OK	TestAccAzureRMSubnet_bug7986 	3m:49s,405ms 	9
OK	TestAccAzureRMSubnet_removeNetworkSecurityGroup 	3m:47s,508ms 	8
OK	TestAccAzureRMSubnet_bug15204 	3m:45s,122ms 	7
OK	TestAccAzureRMSubnetNetworkSecurityGroupAssociation_basic 	3m:41s,959ms 	6
OK	TestAccAzureRMSubnetRouteTableAssociation_basic 	3m:39s,716ms 	5
OK	TestAccAzureRMSubnet_disappears 	3m:33s,881ms 	4
OK	TestAccAzureRMSubnet_serviceEndpoints 	3m:32s,419ms 	3
OK	TestAccAzureRMSubnet_delegation 	3m:27s,983ms 	2
OK	TestAccAzureRMSubnetRouteTableAssociation_deleted 	3m:26s,674ms 	1

[ghost]

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