Make numeric_resolution on date mapping nullable

Author: gmarz

src/Nest/Domain/Mapping/Types/DateMapping.cs

@@ -43,6 +43,6 @@ public DateMapping():base("date")
 		public bool? DocValues { get; set; }
 
 		[JsonProperty("numeric_resolution")]
-		public NumericResolutionUnit NumericResolution { get; set; }
+		public NumericResolutionUnit? NumericResolution { get; set; }
 	}
 }