Fixed Visual Studio Code Analysis warning (CA2202)

CA2202 Do not dispose objects multiple times Object 'memoryStream' can
be disposed more than once in method
'MongoDBSessionStateProvider.SetAndReleaseItemExclusive(HttpContext,
string, SessionStateStoreData, object, bool)'. To avoid generating a
System.ObjectDisposedException you should not call Dispose more than one
time on an object.: Lines: 118 MongoDB.Web
MongoDBSessionStateProvider.cs 118
BsonValue.RawValue is deprecated. Using BsonTypeMapper.MapToDotNetValue instead.
Added StyleCop
Updated .gitignore