Fix retry after, mock specs

In session.rb:
- Fixes setting "retry_after" value on session create
- Fixes setting "expiration_date" when calling "never_expire"
- Fixes setting "session_id" after creating a session
- Adds spec for Session.create when 202 response (Retry) received
- Mocks httparty responses for some tests

I think there are some major problems with this SDK overall that should make you reconsider the design:

- Widespread use of class state/globals (My callers have to clear BoxView::session_id and BoxView::document_id manually before new "create" calls are made)
- Not enough test coverage
- Tests integrated with actual Box API