#79 Implement an interface for mocking in UnitTests
#59, #63 Include response content in WebPushException & Improve the error message when the request fails
#64 Add cancellation taken to Async methods and remove duplicate methods.
#60 Add GCM key when calling FCM endpoint if VapidDetails are not provided.
#71 202 HTTP response causes WebPushException
Allow Vapid header to include expiration.
#53 - Allow HttpClient to be passed via constructor.