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/Podfile.lock

@@ -1,7 +1,7 @@
 PODS:
-  - Alamofire (4.5.0)
+  - Alamofire (4.5.1)
   - TestClient (0.0.1):
-    - Alamofire (~> 4.5)
+    - Alamofire (~> 4.5.0)
 
 DEPENDENCIES:
   - Alamofire (~> 4.5)
@@ -12,8 +12,8 @@ EXTERNAL SOURCES:
     :path: ../
 
 SPEC CHECKSUMS:
-  Alamofire: f28cdffd29de33a7bfa022cbd63ae95a27fae140
-  TestClient: bcecc2c065b23aaa97f5b66a3f96741b560c381b
+  Alamofire: 2d95912bf4c34f164fdfc335872e8c312acaea4a
+  TestClient: c12a94972a1128167637d39ea0b072f46c0790de
 
 PODFILE CHECKSUM: a4351ac5e001fd96f35ed7e647981803864100b5