[R3.1-build-13]: Updating the dns server address for existing subnet through api should throw error if DNS does not get updated

Updating the DNS for subnet may not be allowed just like gateway.
But if we try to update the dns through vnc api, it does NOT throw any error, however dns does not get updated.

new dns server(0.0.0.0) was seen in PUT request to api but it's not updated and does not throw any error.

(Pdb) print vn_obj.network_ipam_refs[0]['attr'].ipam_subnets[0].__dict__
{'subnet': <vnc_api.gen.resource_xsd.SubnetType object at 0x7f2efbc7ca50>, 'addr_from_start': True, 'enable_dhcp': True, 'default_gateway': u'176.105.34.1', 'alloc_unit': 1, 'dns_nameservers': [], 'dhcp_option_list': None, 'subnet_uuid': u'5afcae06-fea0-4ba0-84e8-02c1a90c27ae', 'allocation_pools': [], 'host_routes': None, 'dns_server_address': '0.0.0.0', 'subnet_name': u''}
(Pdb) self.vnc_lib_h.virtual_network_update(vn_obj)
u'{"virtual-network": {"href": "http://10.204.217.104:8082/virtual-network/a79939c9-04fe-4d87-9fd5-0981308c5692", "uuid": "a79939c9-04fe-4d87-9fd5-0981308c5692"}}'

2016 Aug 03 12:33:09.783174 nodec64 [Config:contrail-api:0:__default__][SYS_INFO] : VncApiConfigLog:290 [VncApiCommon: identifier_uuid = a79939c9-04fe-4d87-9fd5-0981308c5692, object_type = virtual_network, identifier_name = default-domain:ctest-DisablePolicyEcmp-96635670:ctest-ctest-DisablePolicyEcmp-96635670-12148927, url = http://10.204.217.104:8082/virtual-network/a79939c9-04fe-4d87-9fd5-0981308c5692, operation = put, useragent = nodec64:/usr/local/lib/python2.7/dist-packages/testtools/run.py, remote_ip = 10.204.217.104:8082, body = {"virtual-network": {"network_ipam_refs": [{"to": ["default-domain", "default-project", "default-network-ipam"], "attr": {"ipam_subnets": [{"subnet": {"ip_prefix": "176.105.34.0", "ip_prefix_len": 24}, "addr_from_start": true, "enable_dhcp": true, "default_gateway": "176.105.34.1", "dns_nameservers": [], "alloc_unit": 1, "subnet_uuid": "5afcae06-fea0-4ba0-84e8-02c1a90c27ae", "dhcp_option_list": null, "host_routes": null, "dns_server_address": "0.0.0.0", "subnet_name": "", "allocation_pools": []}], "host_routes": null}, "uuid": "7deb94c4-885d-4fe0-860e-de356e49e43f"}], "uuid": "a79939c9-04fe-4d87-9fd5-0981308c5692"}}, domain = default-domain, project = ctest-DisablePolicyEcmp-96635670, user = DisablePolicyEcmp]
2016 Aug 03 12:33:09.792236 nodec64 [Config:contrail-api:0:__default__][SYS_INFO] : VncApiStatsLog:4909 [VncApiStats: operation_type = PUT, user = DisablePolicyEcmp, useragent = nodec64:/usr/local/lib/python2.7/dist-packages/testtools/run.py, remote_ip = 10.204.217.104:8082, domain_name = default-domain, project_name = ctest-DisablePolicyEcmp-96635670, object_type = virtual_network, response_time_in_usec = 11823, response_size = 160, resp_code = 200]