Load AlsoLoad field only when primary 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 primary fields exists.

Also add some test fixes.