some error on foreign_key with Version 2.1.7 
This error affected py3k, which I don't regularly test -- my fault.  I've fixed and re-pushed to PYPI as 2.1.7.

When I up to 2.1.7 today, tables con't create 。But in 2.16 I know it works good。

Below is what happened when I use it with Python3 and MySQL.

Traceback (most recent call last):
  File "E:/Work/Python/notepy/model/model.py", line 74, in <module>
    PostTag.create_table()
  File "C:\Python33\lib\site-packages\peewee.py", line 2683, in create_table
    db.create_table(cls)
  File "C:\Python33\lib\site-packages\peewee.py", line 2116, in create_table
    return self.execute_sql(qc.create_table(model_class, safe))
  File "C:\Python33\lib\site-packages\peewee.py", line 1229, in create_table
    return ' '.join(self.create_table_sql(model_class, safe))
  File "C:\Python33\lib\site-packages\peewee.py", line 1223, in create_table_sql
    columns.append(' '.join(foreign_key))
AttributeError: 'map' object has no attribute 'append'
