Fix postgresql_include in create_index

Fix postgresql_include in create_index

Fixed issue where usage of the PostgreSQL ``postgresql_include`` option
within a :meth:`.Operations.create_index` would raise a KeyError, as the
additional column(s) need to be added to the table object used by the
construct internally. The issue is equivalent to the SQL Server issue fixed
in :ticket:`513`. Pull request courtesy Steven Bronson.

Copied from e01041b and 174cb4b.

Fixes: #874
Closes: #875
Pull-request: https://github.com/sqlalchemy/alembic/pull/875
Pull-request-sha: 2f9b51890a1296146bcea842dc8bbee53d1d11f5

Change-Id: I68cc2a75dafd3297c5121c5125ff9c463c74f777