fix result.columns() method

fix result.columns() method

Fixed issue in :meth:`.Result.columns` method where calling upon
:meth:`.Result.columns` with a single index could in some cases,
particularly ORM result object cases, cause the :class:`.Result` to yield
scalar objects rather than :class:`.Row` objects, as though the
:meth:`.Result.scalars` method had been called. In SQLAlchemy 1.4, this
scenario emits a warning that the behavior will change in SQLAlchemy 2.0.

Fixes: #7953
Change-Id: I3c4ca3eecc2bfc85ad1c38000e5990d6dde80d22