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.
The PATCH verb is a tricky beast. Based on the nature of the PATCH  verb I believe it quite reasonable to have all fields nullable (unless the documentation states *required*) so that it may adhere to the PATCh standard (https://developer.github.com/v3/#http-verbs):

> Used for updating resources with partial JSON data. For instance, an Issue resource has title and body attributes. A PATCH request may accept one or more of the attributes to update the resource.

It's not necessarily that a value must be true or false because in a PATCH request a null (or absence of a value/field) is a reasonable request as well. For example: If this was still a non-nullable field it would mean that the serialize would serialize the value whether you set it or not. However, this is not desirable. I, as a user, don't want to be forced in sending data that I don't want to change. This means that if I wanted to update the title of the Release I must also know the previous state of the "Draft" field so that I may maintain it's value.