"nil hack" was not complete.

According to http://docs.recurly.com/api/push-notifications we can meet element with attribute nil="true" and nil attribute can be combined with other (eg nil="true" type="datetime")
(cherry picked from commit 7a340cf36b5d7f85caef48255ed5599fd80dfa66)

This patch removes the RecurlyObject.newXmlMapper method from the original commit and adds more unit tests.

Written-by: Didier Bathily <didier.bathily@njin.fr>
Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
Fix boolean hack
Recurly push notification body serialization

Conflicts:

	src/main/java/com/ning/billing/recurly/RecurlyClient.java
	src/main/java/com/ning/billing/recurly/model/Subscription.java
	src/main/java/com/ning/billing/recurly/model/SubscriptionUpdate.java
	src/test/java/com/ning/billing/recurly/model/TestModelBase.java

Main differences with the original commit (05197d8):

* Updated the copyright date for consistency with the new push model files
* Fixed trivial code style issues
* Moved the introduction of the RecurlyObject.newXmlMapper method from commit 7a340cf

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
RecurlyClient: remove boxing

Reported-by: Didier Bathily <didier.bathily@njin.fr>
Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
Merge branch 'master' into dbathily-push-notifications

Conflicts:
	src/main/java/com/ning/billing/recurly/model/RecurlyObject.java

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
CREDITS: update with push notifications

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>