Update SessionInProcHandler.cs

Reporting "Timeout session optimization" of the master source correct an 
NullReferenceException when ResetItemTimeout() is called.
i.e. https://bugzilla.xamarin.com/show_bug.cgi?id=925

=> in ResetItemTimeout()
cache.Remove (CacheId); call OnSessionRemoved() what do the item.Dispose(),
then InsertSessionItem (inProcItem, itemTimeout, CacheId); insert item with a null rwlock 

So all the next requests crash with the GetItemInternal() :
nullReferenceException on 
"if (item.rwlock.TryEnterUpgradeableReadLock (lockAcquireTimeout))"