Adding key to a date range aggregation

Author: taschmidt

Diff for: src/Nest/DSL/Facets/DateExpressionRange.cs

@@ -9,7 +9,10 @@ public class DateExpressionRange
 		internal string _From { get; set; }
 
 		[JsonProperty(PropertyName = "to")]
-		internal string _To { get; set; }
+		internal string _To { get; set; }
+
+        [JsonProperty(PropertyName = "key")]
+        internal string _Key { get; set; }
 
 		public DateExpressionRange From(string value)
 		{
@@ -20,6 +23,11 @@ public DateExpressionRange To(string value)
 		{
 			this._To = value;
 			return this;
-		}
-	}
+		}
+        public DateExpressionRange Key(string key)
+        {
+            this._Key = key;
+            return this;
+        }
+    }
 }