Add tests for generated models

[maicki]

We should add tests for our generated models to make changes way more safer and see immediately if we break something.

We should look into adding the tests to the Example/ folder and we could swift test for that project.

I ran into the issue in #67 as I wanted to be sure that the model dictionary is the same as calling dictionaryRepresentation on it. The tests could look like something like that:

NSDictionary *imageModelDictionary = @{
    @"height" : @(12),
    @"width" : @(11),
    @"url" : @"http://google.com"
};
Image *image = [[Image alloc] initWithModelDictionary:imageModelDictionary];
XCTAssert([imageModelDictionary isEqualToDictionary:[image dictionaryRepresentation]], @"Should be the same");


NSDictionary *userModelDictionary = @{
    @"id" : @(123),
    @"first_name" : @"Michael",
    @"last_name" : @"Schneider",
    @"image" : imageModelDictionary,
};
User *user = [[User alloc] initWithModelDictionary:userModelDictionary];
XCTAssert([userModelDictionary isEqualToDictionary:[user dictionaryRepresentation]], @"Should be the same");

var imageModelDictionary: [AnyHashable: Any] = ["height": (12), "width": (11), "url": "http://google.com"]
var image = Image(modelDictionary: imageModelDictionary)
XCTAssert(imageModelDictionary.isEqual(to: image.dictionaryRepresentation), "Image dictionary representation should be the same as the model dictionary.")
var userModelDictionary: [AnyHashable: Any] = ["id": (123), "first_name": "Michael", "last_name": "Schneider", "image": imageModelDictionary]
var user = User(modelDictionary: userModelDictionary)
XCTAssert(userModelDictionary.isEqual(to: user.dictionaryRepresentation), "User dictionary representation should be the same as the model dictionary")