date_or_null fix

[tabermike]

fix: expect_json_types when type is :date_or_null and actual value is null
Includes tests to validate change

Dates may be null until specifically set; example response: {"dateDeleted": null}
This patch corrects invalid type failure when response is a valid date; example response:
{"dateDeleted": "2009-12-09T06:00:00Z"} and expected type is :date_or_null

[coveralls]

Coverage increased (+0.1%) to 99.103% when pulling dbd6fef on Widen:date-or-null-fix into 55311ac on brooklynDev:master.

[sethpollack]

Thanks!