Support object schemas with only additionalProperties. (#6492)

Previously, we had implemented the Codable protocol by simply claiming conformance, and making sure that each of our internal classes also implemented the Codable protocol. So our model classes looked like:

class MyModel: Codable {
   var propInt: Int
   var propString: String
}

class MyOtherModel: Codable {
   var propModel: MyModel
}

Previously, our additionalProperties implementation would have meant an object schema with an additionalProperties of Int type would have looked like:

class MyModelWithAdditionalProperties: Codable {
   var additionalProperties: [String: Int]
}

But the default implementation of Codable would have serialized MyModelWithAdditionalProperties like this:

{
  "additionalProperties": {
    "myInt1": 1,
    "myInt2": 2,
    "myInt3": 3
  }
}

The default implementation would put the additionalProperties in its own dictionary (which would be incorrect), as opposed to the desired serialization of:

{
  "myInt1": 1,
  "myInt2": 2,
  "myInt3": 3
}

So therefore, the only way to support this was to do our own implementation of the Codable protocol. The Codable protocol is actually two protocols: Encodable and Decodable.

So therefore, this change generates implementations of Encodable and Decodable for each generated model class. So the new generated classes look like:

class MyModel: Codable {
   var propInt: Int
   var propString: String

   // Encodable protocol methods

   public func encode(to encoder: Encoder) throws {

        var container = encoder.container(keyedBy: String.self)

        try container.encode(propInt, forKey: "propInt")
        try container.encode(propString, forKey: "propString")
    }

    // Decodable protocol methods

    public required init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: String.self)

        propInt = try container.decode(Int.self, forKey: "propInt")
        propString = try container.decode(String.self, forKey: "propString")
    }

}

class MyOtherModel: Codable {
   var propModel: MyModel

   // Encodable protocol methods

   public func encode(to encoder: Encoder) throws {

        var container = encoder.container(keyedBy: String.self)

        try container.encode(propModel, forKey: "propModel")
    }

    // Decodable protocol methods

    public required init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: String.self)

        propModel = try container.decode(MyModel.self, forKey: "propModel")
    }

}

samples/client/test/swift4/default/TestClientApp/Pods/Local Podspecs/TestClient.podspec.json

@@ -13,10 +13,10 @@
   "license": "Proprietary",
   "homepage": "https://github.com/swagger-api/swagger-codegen",
   "summary": "TestClient",
-  "source_files": "TestClient/Classes/Swaggers/**/*.swift",
+  "source_files": "TestClient/Classes/**/*.swift",
   "dependencies": {
     "Alamofire": [
-      "~> 4.5"
+      "~> 4.5.0"
     ]
   }
 }