WNS priorities header support

[sctest]

Hi,

Based on microsoft documentation on WNS Priorities, cloud services can send priority notifications to an app by setting a X-WNS-Priority header in the notification. Can you please add support in java-wns library for this header? It's not listed in WnsNotificationRequestOptional class.

[fernandospr]

Hi @sctest, thanks for your feedback. Please, can you submit a pull request?