Fix postgres detect serial in autogenerate (#1479)

Fix postgres detect serial in autogenerate (#1479)

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Fixes: https://github.com/sqlalchemy/alembic/issues/1479
### Description
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In https://github.com/sqlalchemy/alembic/issues/73, it tries to detact postgresql serial in autogenerate, so it won't take `nextval('seq'::regclass)` as server default for that column.
But it takes not effect for tables not in search path. This PR fixed it.

### Checklist
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This pull request is:

- [ ] A documentation / typographical error fix
	- Good to go, no issue or tests are needed
- [x] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.
- [ ] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

**Have a nice day!**

Closes: #1486
Pull-request: https://github.com/sqlalchemy/alembic/pull/1486
Pull-request-sha: 24df8f906d281df92c531df5a9e1f64d8cdb8527

Change-Id: I50276875bfb1d4f920f0fcd20136337ae09b5384