Handle nullable keys for root folders

The root folder is special and has many null-valued keys
that are normally non-nullable. For such keys, suppress
NSNull and convert it to nil.

This commit adds test coverage for several additional
fields in BoxItem objects.

Bug report: http://stackoverflow.com/questions/19411700/assertion-failure-in-nsjsonserialization-ensureobjectforkeyindictionaryhasex
Merge pull request #47 from box/root-folder-null-keys

Handle nullable keys for root folders