- Added support for parameter-ordered SET clauses in an UPDATE

- Added support for parameter-ordered SET clauses in an UPDATE
statement.  This feature is available by passing the
:paramref:`~.sqlalchemy.sql.expression.update.preserve_parameter_order`
flag either to the core :class:`.Update` construct or alternatively
adding it to the :paramref:`.Query.update.update_args` dictionary at
the ORM-level, also passing the parameters themselves as a list of 2-tuples.
Thanks to Gorka Eguileor for implementation and tests.
adapted from pullreq github:200

(cherry picked from commit c90f0a49f332867f6b337c79ddf192299788667f)