All models used in PATCH verbs should have nullable fields

There were a few update models, used in PATCH requests, which had fields that were not nullable. This pull requests makes sure all fields within PATCH models can null every field (unless otherwise specified by the API docs).

1. Cloned the ReleaseUpdate (used in PATCH requests) model into a NewRelease model (used in PUT requests) since the two models should not have been shared between the request types as they had different field requirements.
2. Cleaned up the PullRequestUpdate object as it looks like there was a copy and paste mistake since it contained a few too many fields that don't exist on the update.
This follows the other comment I made about the importance that the PATCH model object be completely nullable with the exception of the *required* fields. In this case, there is a drastic difference between the labels field being null and it being an empty list. For example: If I want to change the title of an issue and only the title when I instantiate this `IssueUpdate` model it automatically sets the `Labels` to an empty list. When I send the request, the serializer will serialize the request as:

```{"Name": "New Name", "Labels": []}```

which GitHub will interpret as "i want this issue to have no labels". However, what really should have happened is that, because the "Labels" was null, the serialize did not serialize the field:

```{"Name": "New Name"}```

and thus GitHub did not modify the Labels resource on the other side and only modified the title.