Load AlsoLoad field only when general field does not exists

In [docs](http://docs.doctrine-project.org/projects/doctrine-mongodb-odm/en/latest/reference/annotations-reference.html#alsoload) described:
The above $fieldName will be loaded from fieldName if it exists and will fallback to oldFieldName if it does not exist.
But now AlsoLoad loads field even when general fields exists.

Also add some test fixes.