Add a few tests for FirestoreDocument and fix odd behaviour of getCre…

…atedTime (ahsankhatri#15)

Added few test cases.

Store http response code in UnhandledRequestError (ahsankhatri#12)

This allows the user to deal with errors in a more informed way - for example, if the response code is 502 or 503, it might be a good idea to wait 15-30 seconds and try again.

Fix crash when trying to read empty array (ahsankhatri#4)

* Fix crash when trying to read empty array

First/Initial release.