Fix LazilyDecoding not handling top-level optionals

Tusker/LazilyDecoding.swift

@@ -37,10 +37,10 @@ public struct LazilyDecoding<Enclosing, Value: Codable> {
             } else {
                 guard let data = instance[keyPath: wrapper.keyPath] else { return wrapper.fallback }
                 do {
-                    let value = try decoder.decode(Value.self, from: data)
+                    let value = try decoder.decode(Box<Value>.self, from: data)
-                    wrapper.value = value
+                    wrapper.value = value.value
                     instance[keyPath: storageKeyPath] = wrapper
-                    return value
+                    return value.value
                 } catch {
                     return wrapper.fallback
                 }
@@ -50,7 +50,7 @@ public struct LazilyDecoding<Enclosing, Value: Codable> {
             var wrapper = instance[keyPath: storageKeyPath]
             wrapper.value = newValue
             instance[keyPath: storageKeyPath] = wrapper
-            let newData = try? encoder.encode(newValue)
+            let newData = try! encoder.encode(Box(value: newValue))
             instance[keyPath: wrapper.keyPath] = newData
         }
     }
@@ -62,3 +62,11 @@ extension LazilyDecoding {
         self.init(from: keyPath, fallback: [] as! Value)
     }
 }
+
+extension LazilyDecoding {
+    // PropertyListEncoder only allows top-level types to be dicts or arrays, which breaks encoding nil-able values.
+    // Wrapping everything in a Box ensures that it's always a dict.
+    private struct Box<T: Codable>: Codable {
+        let value: T
+    }
+}