Date histogram agg descriptor now accepts DateInterval enum

Closes #981

Author: gmarz

Diff for: src/Nest/DSL/Aggregations/DateHistogramAggregationDescriptor.cs

@@ -143,6 +143,13 @@ public DateHistogramAggregationDescriptor<T> Interval(string interval)
 			Self.Interval = interval;
 			return this;
 		}
+
+		public DateHistogramAggregationDescriptor<T> Interval(DateInterval interval)
+		{
+			var intervalString = interval.GetStringValue();
+			Self.Interval = intervalString;
+			return this;
+		}
 
 		public DateHistogramAggregationDescriptor<T> Format(string format)
 		{

Diff for: src/Tests/Nest.Tests.Integration/Aggregations/BucketAggregationTests.cs

@@ -232,7 +232,7 @@ public void DateHistogram()
 				.Aggregations(a => a
 					.DateHistogram("bucket_agg", dh => dh
 						.Field(p => p.StartedOn)
-						.Interval("1d")
+						.Interval(DateInterval.Day)
 					)
 				)
 			);