Next available IP Address of a /31 prefix

[avolmensky]

Environment

• Python version: 2.7.13
• NetBox version: 2.5.2

Steps to Reproduce

When requesting next available IP from the API within a prefix that is a /31, an empty list is returned. This is causing issues further down the track with pynetbox not being able to create an IP address in a /31 prefix.

I have tracked this down to line 442 of ipam/models.py https://github.com/digitalocean/netbox/blob/f7f6704fc1956427608c86870f4bd609df0fb8ef/netbox/ipam/models.py#L442
According to RFC3021 first and last address in a /31 is a usable IP Address.

The following changes fix the problem. Should I create a PR for this?

From:

        if not self.is_pool:

To:

        if not self.is_pool and not (self.prefix.prefixlen == 31 and self.family == 4):