Make PropType.object pass for Date and RegExp instances

8855d6153e252c735de0e6cc373787d22c1a467b gave more accurate error messages for date and regexp by returning 'date' and 'regexp' respectively from `getPropType`.
However, for the object primitive check, which compares the instance passed with `getPropType's` return string, `object` !== 'date'.
This adds special hanlding for those.
add displayName to component spec functions

closes #992