Fix serialization of Occurance and FutureOccurance

[tugberkugurlu]

Fixes #36.

[tugberkugurlu]

bc5790e is the repro of this, fails with below error:

AspNetCore.Identity.MongoDB.Tests.MongoIdentityUserTests.MongoIdentityUser_ShouldSaveAndRetrieveTheFutureOccuranceCorrectly

Assert.Equal() Failure
Expected: FutureOccurrence { Instant = 2017-02-01T00:00:00.8996910Z }
Actual:   FutureOccurrence { Instant = 2017-02-01T00:00:00.8990000Z }
   at Xunit.Assert.Equal[T](T expected, T actual, IEqualityComparer`1 comparer)
   at AspNetCore.Identity.MongoDB.Tests.MongoIdentityUserTests.<MongoIdentityUser_ShouldSaveAndRetrieveTheFutureOccuranceCorrectly>d__1.MoveNext() in /Users/tugberk/apps/AspNetCore.Identity.MongoDB/tests/AspNetCore.Identity.MongoDB.Tests/MongoIdentityUserTests.cs:line 71
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Xunit.Sdk.TestInvoker`1.<>c__DisplayClass48_1.<<InvokeTestMethodAsync>b__1>d.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Xunit.Sdk.ExecutionTimer.<AggregateAsync>d__4.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Xunit.Sdk.ExceptionAggregator.<RunAsync>d__9.MoveNext()

see https://travis-ci.org/tugberkugurlu/AspNetCore.Identity.MongoDB/builds/196766190#L2409

[tugberkugurlu]

have a look: https://alexmg.com/datetime-precision-with-mongodb-and-the-c-driver/

[tugberkugurlu]

Also see http://stackoverflow.com/questions/6036433/datetime-issues-with-mongo-and-c-sharp

[tugberkugurlu]

see http://mongodb.github.io/mongo-csharp-driver/2.4/reference/bson/mapping/

[tugberkugurlu]

I am not sure why the build is hanging for this, could be related to using multiple MongoClient per app (not entirely sure). LGTM and worry about it on master if it keeps happening.