Fixed issue with aliases and deep = True

In BaseLoader.construct_object constructed_objects were being returned without checking if they'd been generated with deep=True.

Test:
>>> import yaml
>>> def test(loader, node):
>>>     return loader.construct_sequence(node, deep=True)
>>> yaml.add_constructor('!test', test)
>>> print(yaml.load("""
- &a [anchored]
- !test [*a]
""")

Expected output: [['anchored'], [['anchored']]]

Output before: [['anchored'], [[]]]