public void TestToString()
 {
     var wrapper = new BsonDocumentWrapper(new BsonDocument("x", 1));
     Assert.AreEqual(false, wrapper.IsMaterialized);
     var expected = "{ 'x' : 1 }".Replace("'", "\"");
     var s = wrapper.ToString();
     Assert.AreEqual(false, wrapper.IsMaterialized); // ToString just serializes, doesn't materialize
     Assert.AreEqual(expected, s);
 }