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
  {