Allocation from NeutronSubnet_IPAllocation pool fails when starting with two addresses

If an allocation pool has two addresses in it (e.g. start: "10.10.10.10" and end: "10.10.10.11") then allocating an address returns 10.10.10.10, but leaves the pool in the state (start: null, end: "10.10.10.11")

Correct behavior is start and end should both be "10.10.10.11"