[Go] Readonly properties on PUT methods

[mcardosos]

Currently, the Go SDK includes some parameter validation. If readonly parameters (like provisioning state) are passed, the function will return an error.
This is not as cool when you have lines like this, in which the same struct you got in a GET operation will be passed in a PUT operation (let's say I just want to update data disk size)...

	vm, err := vmClient.Get(resourceGroupName, vmName, compute.InstanceView)
        (*vm.Properties.StorageProfile.DataDisks)[index].DiskSizeGB = to.Int32Ptr(newSize)
	vm.Properties.ProvisioningState = nil
	vm.Properties.InstanceView = nil
	vm.Properties.VMID = nil
	_, err = vmClient.CreateOrUpdate(resourceGroupName, vmName, vm, nil)

Setting all these readonly parameters to nil looks horrible :(
Maybe it could be done inside the function (in this case CreateOrUpdate).
Thoughts? cc @jhendrixMSFT @vishrutshah

[mcardosos]

Fixed in #1749