Correctly deserialize Percentiles aggregate (#3724)

This commit correctly deserializes percentiles aggegations by closing
the object or array in which values are returned. In addition, percentile item values can be
nullable, so read as null or double and update PercentileItem to have nullable value.

Closes #3715

Author: russcam

src/Nest/Aggregations/AggregateFormatter.cs

@@ -325,9 +325,11 @@ private IAggregate GetPercentilesAggregate(ref JsonReader reader)
 					metric.Items.Add(new PercentileItem
 					{
 						Percentile = double.Parse(propertyName),
-						Value = reader.ReadDouble()
+						Value = reader.ReadNullableDouble()
 					});
 				}
+
+				reader.ReadNext(); // }
 			}
 			else
 			{
@@ -343,10 +345,12 @@ private IAggregate GetPercentilesAggregate(ref JsonReader reader)
 					metric.Items.Add(new PercentileItem
 					{
 						Percentile = percentile,
-						Value = reader.ReadDouble()
+						Value = reader.ReadNullableDouble()
 					});
 					reader.ReadNext(); // }
 				}
+
+				reader.ReadNext(); // ]
 			}
 
 			return metric;

src/Nest/Aggregations/Metric/Percentiles/PercentilesMetricAggregate.cs

@@ -5,7 +5,7 @@ namespace Nest
 	public class PercentileItem
 	{
 		public double Percentile { get; internal set; }
-		public double Value { get; internal set; }
+		public double? Value { get; internal set; }
 	}
 
 	public class PercentilesAggregate : MetricAggregateBase