v0.4.12 breaks mixed dict with enum if model-as-string=true

[lmazuel]

This breaks:

        async_security_rule = self.network_client.security_rules.create_or_update(
            self.group_name,
            security_group_name,
            new_security_rule_name,
            {
                    'access':azure.mgmt.network.models.SecurityRuleAccess.allow,
                    'description':'New Test security rule',
                    'destination_address_prefix':'*',
                    'destination_port_range':'123-3500',
                    'direction':azure.mgmt.network.models.SecurityRuleDirection.outbound,
                    'priority':400,
                    'protocol':azure.mgmt.network.models.SecurityRuleProtocol.tcp,
                    'source_address_prefix':'*',
                    'source_port_range':'655',
            }
        )