azure_rm_virtualmachineextension always changed

An `azure_rm_virtualmachineextension` always reports as changed even when nothing changes.

I have used this module to make 3 extensions and 2 are always listed as having changes-- the two that use `protected_settings`. I noticed in the output it shows `state.protected_settings` is null even though I am setting that in my ansible config.

To reproduce:
* Try to add the `JsonADDomainExtension` to a windows VM, passing the password as a `protected_setting`
* Don't modify anything and repeat -- it still reports changes.

e.g.
```yaml
- name: "Create VM Extension to join AD"
delegate_to: localhost
tags:
- vm
- base
vars:
extension_settings:
Name: "{{ domain_name }}"
User: "{{ domain_admin_user }}"
Restart: "true"
Options: "3"
OUPath: "{{ domain_server_ou }}"
protected_extension_settings:
Password: "{{ domain_admin_password }}"
azure_rm_virtualmachineextension:
name: "join-domain"
virtual_machine_extension_type: "JsonADDomainExtension"
resource_group: "{{ resource_group_vm }}"
virtual_machine_name: "{{ inventory_hostname }}"
publisher: "Microsoft.Compute"
type_handler_version: "1.3"
auto_upgrade_minor_version: true
settings: "{{ extension_settings | to_json }}"
protected_settings: "{{ protected_extension_settings | to_json }}"
```

I'm guessing that the Azure API doesn't return `protected_settings` so when the code looks for a change ([right about here](https://github.com/Azure/azure_modules/blob/master/library/azure_rm_virtualmachineextension.py#L239)) it is always different.