Fix two serialization issues

Hi there!
One commit to fix OauthToken.Scope not correctly being deserialized (issue #582). I understand that the original code was able to serialize/deserialize list into comma separated values. 

Another one is when deserializing a bool that can be null (It happens when receiving a webhook callback with mergeable property that is "bool" but had a null value in the payload). Again, not a big issue, as I finally opted to use json.net to deserialize the data in a safer way and don't really use this fix anymore (I don't want the deserialization process to fail because it is not correctly mapped to the language and just need to access some specific properties in the payload)

So probably this pull request cannot be merged, but I I just keep this fix here at least to leave a reminder of the problem on your side (and keep a working code on my branch).