_custom_field_processing bug in NetworkV4 fails to properly serialize members

[ddiguru]

High-level API call to NetworkV4 fails to serialize members properly. This has been confirmed on the infoblox-client 0.5.0, as well as, master. This was tested and confirmed on NIOS v8.5.3 and v8.5.4.

Steps to reproduce:

1. create a DHCP-enabled network 192.168.1.0/24 and associate two (2) Grid DHCP Member servers (NOT Microsoft Servers)

2. perform the following call in the Infoblox client: res = objects.Network.search(conn, return_fields=['members'], network='192.168.1.0/24')

The following is returned:

NetworkV4: members="[Msdhcpserver: ipv4addr="192.168.1.5", name="dhcp01.example.com", 
Msdhcpserver: ipv4addr="192.168.1.6", name="dhcp02.example.com"]", 
_ref="network/ZG5zLm5ldHdvcmskMTAuMC4zMi4wLzI0LzA:192.168.1.0/24/default"

There are a few issues with this:

• _struct is missing from the list where the value is either 'msdhcpserver' | 'dhcpmember'
• It's interpreting the members as Msdhcpserver objects 100% of the time instead of using _struct to determine the type of DHCP server that is associated with the network

This issue is also seen with the NetworkTemplate.

This issue is only related to the high-level calls to objects.py and NOT when using the connector low-level calls.

[ddiguru]

created PR #295