fix rendering of foreign key constraints when using a metadata schema

without the fix i got the following error when generating a new revision:
```python
Traceback (most recent call last):
  ...
  File ".../alembic/alembic/autogenerate/render.py", line 386, in _fk_colspec
    tokens = fk._colspec.split(".")
  File ".../sqlalchemy/sql/elements.py", line 623, in __getattr__
    key)
AttributeError: Neither 'Column' object nor 'Comparator' object has an attribute 'split'
```