Add type and site_admin fields to GHPerson

[martinvanzijl]

Description

Helps with #126. This adds the type and site_admin field from the API to GHPerson. It does not completely solve the issue, but at least you can use getType() to see whether the owner is actually a User or an Organization.

NOTE: I'm not sure I need the calls to populate(); it worked fine without them in my unit tests. How can I be sure?

[bitwiseman]

@martinvanzijl
Short answer: I don't know but what you have done is a reasonable start.

Slightly longer answer: populate() is needed for when the User or Organization is created from a response that has only partial information - but I don't know which fields those are. 😭

Non-answer information: I started to add the Refreshable interface that will work without knowing which fields are missing from partial responses. However, it isn't applied generally yet and it is basically the same as populate() - which I'm not sure it is the right way to go long term (see #589 for some background).