Update spec to be a more closer representation of the server responses

[brismithers]

The following changes were made to the spec, to be more closely aligned with what the OneSignal REST API is returning:

• Add nullable to properties which may be null. I do not believe this is an exhaustive list, but are ones that I immediately identified as fields that could be null.
• No longer require properties that are not always returned by the server. Some models are used in more than one endpoint, a property's "required-ness" can depend on the endpoint. When this is the case, I removed the required attribute from these properties to support proper serialization/deserialization within the server SDKs.
• Add more accurate data types/formats.