reset _autoincrement_column when table pk columns are replaced

[Mike Bayer]

from sqlalchemy import *

m = MetaData()

t = Table('t', m, 
    Column('id', Integer, primary_key=True)
)

assert t._autoincrement_column is t.c.id

t = Table('t', m, 
    Column('id', Integer, primary_key=True),
    extend_existing=True
)
assert t._autoincrement_column is t.c.id

[Mike Bayer]

152163b3336970460b605fc2a79766d94f467f8d

1a03276d199b1af3f7a301cb84e488cbecc1ccce

[Mike Bayer]

Removing milestone: 0.7.9 (automated comment)