Default date histogram format to date_optional_time

This is a better fix for #955.  The previous fix, which was to leave the
format empty, breaks deserialization to HistogramItem on 1.0.x versions
of ES.  This is because in later versions of ES, key_as_string is returned
in the response by default, whereas in ES 1.0.x, it's only returned if a
format is specified.

Author: gmarz

src/Nest/DSL/Aggregations/DateHistogramAggregationDescriptor.cs

@@ -109,6 +109,11 @@ public class DateHistogramAggregationDescriptor<T> : BucketAggregationBaseDescri
 
 		IDictionary<string, object> IDateHistogramAggregator.ExtendedBounds { get; set; }
 
+		public DateHistogramAggregationDescriptor()
+		{
+			this.Self.Format = "date_optional_time";
+		}
+
 		public DateHistogramAggregationDescriptor<T> Field(string field)
 		{
 			Self.Field = field;