Issue title: Crash Mapper initializer when running in a XCTestCase
 Issue body: ### Your JSON dictionary:

```json
{
  "id": 123,
  "name": "Pedro"
}
```

### Your model:

```swift
@objc
final class User: RealmSwift.Object, Mappable {

  var id: Int = -1
  var name: String = ""
  
  required convenience init?(map: Map) {
    self.init()
  }

  init(_ map: Map) {
    id <- map["id"]
    name <- map["name"]
  }
}
```

### What you did:

```swift
let users = Mapper<User>().mapArray(JSONString: string)
```

### What you expected:

I exepected something like:

```swift
[User]
```

### What you got:

![Screenshot 2021-04-10 at 01 23 43](https://user-images.githubusercontent.com/5521682/114252213-fe475380-999b-11eb-8ad0-411876a88bd7.png)

The crash happens when the unit test calls `Mapper<User>()`, I tried to replace it with `Mapper<User>(shouldIncludeNilValues: true)` and tried to remove `final` from the class `User` but no success.

Xcode: 12.4
Cocoapods: 1.10.1 (It's not using `use_frameworks!` nor `use_modular_headers!`)
Swift: 5.3.2
 Comments: 
 Comment 0: @ppamorim, have you solved this problem? I have the same issue when ```Mapper<Object>``` works anywhere except test classes.
 Comment 1: @crownvic Not yet, We cannot use the library anymore. We are migrating to `Decodable`.
{'source': 'GithubIssues', 'repo_name_with_owner': 'tristanhimmelman/ObjectMapper', 'issue_id': 854937907, 'repo_stars': 9001.0}