Validation fix and API call

[qqmyers]

The two commits fix a misconception in the validation routine and then add an api call to test with. I haven't done more than compile this so there could still be bugs. The api call should allow requests POSTing a json object with
"url" - the unsigned url you want (must be exact with any dataset/file id, etc.
"method" - optional - GET is assumed
"timeout" - optional - 10 minutes is assumed
"userId" - optional - the user id string - assumes the superuser using this API.

[coveralls]

Coverage decreased (-0.006%) to 18.867% when pulling 55fafa5 on GlobalDataverseCommunityConsortium:7715-signed-urls-for-external-tools into 7e82009 on IQSS:7715-signed-urls-for-external-tools.

[qqmyers]

Also added the JSONUtil methods I've mentioned that make it easy to get JsonObject/JsonArray from a string. The prettyprint methods are already there to go the other way.