[subnet] update subnet from horizon will remove default gateway of the subnet

[cj-chung]

Reproduce steps:

1. Create a subnet with default gateway (without clicking on "Disable Gateway")
2. Edit a created subnet from OpenStack Horizon, change the subnet's name only then submit.
3. The default gateway is gone in the list.
4. Check the subnet again, you will see the "Disable Gateway" option is enabled.

The "Default Gateway" option in the Subnet's updating API is set to true when updating a subnet.

[xieus]

The input in the scenario of disabled gateway and no input value for gateway appears to be no difference for Java. Need more investigation.

[cj-chung]

@chenpiaoping add this annotation before the method.

@JsonInclude(JsonInclude.Include.NON_ABSENT)

@kevin-zhonghao, maybe we can try this in the subnet entity.