CSHARP-4238: Convert unit tests for $dateXyz operators to integration tests.

Author: rstam

src/MongoDB.Bson/BsonUtils.cs

@@ -83,7 +83,7 @@ public static DateTime ToDateTimeFromMillisecondsSinceEpoch(long millisecondsSin
                 millisecondsSinceEpoch > BsonConstants.DateTimeMaxValueMillisecondsSinceEpoch)
             {
                 var message = string.Format(
-                    "The value {0} for the BsonDateTime MillisecondsSinceEpoch is outside the" +
+                    "The value {0} for the BsonDateTime MillisecondsSinceEpoch is outside the " +
                     "range that can be converted to a .NET DateTime.",
                     millisecondsSinceEpoch);
                 throw new ArgumentOutOfRangeException("millisecondsSinceEpoch", message);

src/MongoDB.Driver/Linq/DateTimeExtensions.cs

@@ -235,7 +235,7 @@ public static long Subtract(this DateTime @this, DateTime startDate, DateTimeUni
         /// <param name="timezone">The timezone.</param>
         /// <returns>The resulting DateTime.</returns>
         /// <remarks>See the server documentation for $dateDiff for information on timezones in MongoDB.</remarks>
-        public static DateTime Subtract(this DateTime @this, DateTime value, string timezone)
+        public static TimeSpan Subtract(this DateTime @this, DateTime value, string timezone)
         {
             throw new InvalidOperationException("This DateTime.Subtract method is only intended to be used in LINQ queries.");
         }

src/MongoDB.Driver/Linq/Linq3Implementation/Translators/ExpressionToAggregationExpressionTranslators/MethodTranslators/DateTimeSubtractWithDateTimeMethodToAggregationExpressionTranslator.cs

@@ -91,7 +91,7 @@ public static AggregationExpression Translate(TranslationContext context, Method
                     {
                         startOfWeek = null;
                     }
-                    serializer = BsonInt64Serializer.Instance;
+                    serializer = Int64Serializer.Instance;
                 }
                 else
                 {