restore statement substitution to before_execute()

restore statement substitution to before_execute()

Fixed issue where the ability of the
:meth:`_engine.ConnectionEvents.before_execute` method to alter the SQL
statement object passed, returning the new object to be invoked, was
inadvertently removed. This behavior has been restored.

The refactor in a1939719a652774a437f69f8d4788b3f08650089 removed this
feature for some reason and there were no tests in place to detect
it.  I don't see any indication this was planned.

Fixes: #6913
Change-Id: Ia77ca08aa91ab9403f19a8eb61e2a0e41aad138a