Add specs for testing labelField & uniqueIdField

[jackkoppa]

As of #75 (which was a cherry-pick of #53), we will begin allowing users to set:

1. a labelField prop, to identify which property on their options should be used for display
2. a uniqueIdField prop, to identify which property on their options is guaranteed to be unique, and thus can be used for tracking selected options

We now need to ensure there are functional specs in place for those props