Don't use Serializer context

Relates:elastic/elasticsearch-net#4791

This commit updates the AttributesTableConverter and FeatureConverter
to not use the serializer context when deserializing AttributesTable.
The serializer context is not safe to use multithreaded, which can be
the case when JsonConverters from NetTopologySuite.IO.GeoJson are added
to an singleton instance of JsonSerializer not under the control
of NetTopologySuite.IO.GeoJson.

.gitignore

@@ -203,3 +203,4 @@ NetTopologySuite.Samples.Shapefiles/tmp*.shx
 NetTopologySuite.Samples.Shapefiles/tmp*.dbf
 NetTopologySuite.Samples.Shapefiles/test_arcview.shp
 NetTopologySuite.Samples.Shapefiles/test_buffer.shp
+.idea/

src/NetTopologySuite.IO.GeoJSON/Converters/AttributesTableConverter.cs

@@ -148,20 +148,10 @@ private static IList<object> InternalReadJsonArray(JsonReader reader, JsonSerial
             // Advance reader
             reader.Read();
             reader.SkipComments();
-
-            IAttributesTable attributesTable = null;
-            if (!innerObject && serializer.Context.Context is IFeature feature)
-            {
-                attributesTable = feature.Attributes;
-            }
+            var attributesTable = new AttributesTable();
 
             if (reader.TokenType != JsonToken.Null)
             {
-                if (attributesTable is null)
-                {
-                    attributesTable = new AttributesTable();
-                }
-
                 while (reader.TokenType == JsonToken.PropertyName)
                 {
                     string attributeName = (string)reader.Value;

src/NetTopologySuite.IO.GeoJSON/Converters/FeatureConverter.cs

@@ -173,10 +173,23 @@ public override object ReadJson(JsonReader reader, Type objectType, object exist
                                 throw new ArgumentException("Expected token '{' not found.");
                             }
 
-                            var context = serializer.Context;
-                            serializer.Context = new StreamingContext(serializer.Context.State, feature);
-                            feature.Attributes = serializer.Deserialize<AttributesTable>(reader);
-                            serializer.Context = context;
+                            var attributes = serializer.Deserialize<AttributesTable>(reader);
+
+                            if (feature.Attributes is null)
+                            {
+                                feature.Attributes = attributes;
+                            }
+                            else
+                            {
+                                foreach (var attribute in attributes)
+                                {
+                                    if (!feature.Attributes.Exists(attribute.Key))
+                                    {
+                                        feature.Attributes.Add(attribute.Key, attribute.Value);
+                                    }
+                                }
+                            }
+
                             if (reader.TokenType != JsonToken.EndObject)
                             {
                                 throw new ArgumentException("Expected token '}' not found.");