GcmNotificationPriority serialization

[grishin]

What version of PushSharp are you using?

4.0.4

Describe your issue:

I'm having trouble sending pushes to Android 6.0 phone with "high" priority setting. In the code (https://github.com/Redth/PushSharp/blob/master/PushSharp.Google/GcmNotification.cs#L142) i've found that GcmNotificationPriority enum is serialized as int (5, 10) not as strings ("normal", "high") as google's docs require https://developers.google.com/cloud-messaging/concept-options#setting-the-priority-of-a-message.

[JsonProperty ("priority"), JsonConverter(typeof(StringEnumConverter))]
public GcmNotificationPriority? Priority { get; set; }

What are the steps required to reproduce this issue?

Please provide any Exception Stack Traces