Set Format when Missing is specified

Closes #2864

Author: russcam

src/Nest/Aggregations/Bucket/DateHistogram/DateHistogramAggregation.cs

@@ -57,15 +57,12 @@ public class DateHistogramAggregation : BucketAggregationBase, IDateHistogramAgg
 
 		public string Format
 		{
-			get
-			{
-				return !string.IsNullOrEmpty(_format) &&
-					!_format.Contains("date_optional_time") &&
-					ExtendedBounds != null
-					? _format + "||date_optional_time"
-					: _format;
-			}
-			set { _format = value; }
+			get => !string.IsNullOrEmpty(_format) &&
+			       !_format.Contains("date_optional_time") &&
+			       (ExtendedBounds != null || Missing.HasValue)
+				? _format + "||date_optional_time"
+				: _format;
+			set => _format = value;
 		}
 
 		public int? MinimumDocumentCount { get; set; }
@@ -102,15 +99,12 @@ public class DateHistogramAggregationDescriptor<T>
 
 		string IDateHistogramAggregation.Format
 		{
-			get
-			{
-				return !string.IsNullOrEmpty(_format) &&
-					!_format.Contains("date_optional_time") &&
-					Self.ExtendedBounds != null
-					? _format + "||date_optional_time"
-					: _format;
-			}
-			set { _format = value; }
+			get => !string.IsNullOrEmpty(_format) &&
+			       !_format.Contains("date_optional_time") &&
+			       (Self.ExtendedBounds != null || Self.Missing.HasValue)
+				? _format + "||date_optional_time"
+				: _format;
+			set => _format = value;
 		}
 
 		int? IDateHistogramAggregation.MinimumDocumentCount { get; set; }

src/Tests/Aggregations/Bucket/DateHistogram/DateHistogramAggregationUsageTests.cs

@@ -15,8 +15,8 @@ namespace Tests.Aggregations.Bucket.DateHistogram
 	 * From a functionality perspective, this histogram supports the same features as the normal histogram.
 	 * The main difference is that the interval can be specified by date/time expressions.
 	 *
-	 * NOTE: When specifying a `format` **and** `extended_bounds`, in order for Elasticsearch to be able to parse
-	 * the serialized `DateTime` of `extended_bounds` correctly, the `date_optional_time` format is included
+	 * NOTE: When specifying a `format` **and** `extended_bounds` or `missing`, in order for Elasticsearch to be able to parse
+	 * the serialized `DateTime` of `extended_bounds` or `missing` correctly, the `date_optional_time` format is included
 	 * as part of the `format` value.
 	 *
 	 * Be sure to read the Elasticsearch documentation on {ref_current}/search-aggregations-bucket-datehistogram-aggregation.html[Date Histogram Aggregation].