Fix model serialization on Linux (#12)

* Fix model serialization on Linux

* Some more comments, while we're at it

Author: nighthawk

Sources/GeoJSONKit/GeoJSON+Codable.swift

@@ -118,7 +118,11 @@ extension GeoJSON.Feature {
     var encoder = JSONEncoder()
     configure(&encoder)
     let data = try encoder.encode(model)
-    let asDict = try JSONSerialization.jsonObject(with: data) as? [String: AnyHashable]
+    let decoded = try JSONSerialization.jsonObject(with: data)
+    
+    // This is needed for Linux as JSONSerialization behaves differently there
+    let asDict = (decoded as? [String: Any])?.compactMapValues(Adjuster.hashable(_:))
+    
     self.init(geometry: geometry, properties: asDict, id: id)
   }
 

Sources/GeoJSONKit/GeoJSON.swift

@@ -296,7 +296,10 @@ public struct GeoJSON: Hashable {
   /// Features in GeoJSON contain a Geometry object and additional properties.
   public struct Feature: Hashable {
     public var geometry: GeometryObject
+    
+    /// Optional properties, as a dictionary.
     public var properties: [String: AnyHashable]?
+    
     public var id: AnyHashable? // number or string
 
     public init(geometry: GeometryObject, properties: [String: AnyHashable]? = nil, id: AnyHashable? = nil) {
@@ -315,6 +318,9 @@ public struct GeoJSON: Hashable {
       id = dict["id"] as? AnyHashable
     }
 
+    /// - Warning: If there are no properties, the `properties` field will be omitted, to avoid dealing
+    ///     with `nil` values in a dictionary. This is strictly speaking not compliant with the [GeoJSON spec](https://datatracker.ietf.org/doc/html/rfc7946#section-3.2),
+    ///     which says this value should be present with a JSON null value.
     public func toJSON(prune: Bool = true) -> [String: AnyHashable] {
       var json: [String: Any] = [
         "type": "Feature",
@@ -539,7 +545,7 @@ fileprivate extension Array where Element == GeoJSON.Degrees {
   }
 }
 
-fileprivate enum Adjuster {
+enum Adjuster {
   static func prune(_ value: Any) -> Any {
     if let dict = value as? [String: Any] {
       return dict.mapValues(prune(_:))

Tests/GeoJSONKitTests/GeoJSONCodableTest.swift

@@ -206,6 +206,7 @@ final class GeoJSONCodableTest: XCTestCase {
     let model = Model(name: "Test", number: 3517, date: Date())
     let feature = try GeoJSON.Feature(geometry: point, model: model)
     XCTAssertNotNil(feature)
+    XCTAssertNotNil(feature.properties)
 
     let restored = try feature.model(as: Model.self)
     XCTAssertEqual(model, restored)