Memory Overflow at Doctrine Module with related entities

If an entity is on the `1` side of a `1:n` relationship (i.e. it has `oneToMany`), and I handle it with the Doctrine Module, the object gets infinitely large, thereby causing a PHP memory overflow.

### Details

I've set up these two entities:
```yml
AppBundle\Entity\TestGroup:
type: entity
table: test_group

id:
id:
type: integer
generator:
strategy: IDENTITY

fields:
name:
type: string
length: 50
nullable: true

oneToMany:
test_user:
targetEntity: TestUser
mappedBy: test_group
```

```yml
AppBundle\Entity\TestUser:
type: entity
table: test_user

id:
id:
type: integer
generator:
strategy: IDENTITY

fields:
group_id:
type: integer
nullable: true
name:
type: string
length: 50
nullable: true

manyToOne:
test_group:
targetEntity:
inversedBy: test_user
joinColumn:
name: group_id
referencedColumnName: id
```
Now I run this functional test:
```php
public function tryMemoryLeak (FunctionalTester $I)
{
$group = new \AppBundle\Entity\TestGroup();
var_dump($group);
$I->persistEntity($group);
die;
var_dump($group);
}
```
Result: The output from `var_dump` is normal; and the entity exists in the database. But as soon as I remove `die;` the bug occurs: I get endless (i.e. hundreds or thousands of lines) output in the console. The same happens if I fetch the entity from the database with `$I->grabEntityFromRepository`.
This explains what I initially observed: If I try to pass the object to some function, I get a memory overflow crash from PHP.

I've tried it with SQLite and PostgreSQL.

If I do the same with the `TestUser` entity, everything is normal. So I'm figuring it has something to do with the `oneToMany` setting.

And if I do it in a controller, everything is fine too - so I'm figuring there's no error in the entities.

### Setup

* Codeception version: 2.3.5
* PHP Version: 7.1
* Operating System: Windows 10