fix AlterColumnOp modify_name ignorance
Fixed issue where the "modified_name" of :class:`.AlterColumnOp` would not
be considered when rendering op directives for autogenerate. While
autogenerate cannot detect changes in column name, this would nonetheless
impact approaches that made use of this attribute in rewriter recipes. Pull
request courtesy lenvk.
Fixes: #1635
Closes: #1636
Pull-request: https://github.com/sqlalchemy/alembic/pull/1636
Pull-request-sha:
dd103354dcf9cd8db30c66a9a4d3879e4f1c5499
Change-Id: I3e4976a3e703ecd8d69ec208b2d225a8bf6a8251