Provided a new attribute for :class:`.TypeDecorator`

Provided a new attribute for :class:`.TypeDecorator`
called :attr:`.TypeDecorator.coerce_to_is_types`,
to make it easier to control how comparisons using
``==`` or ``!=`` to ``None`` and boolean types goes
about producing an ``IS`` expression, or a plain
equality expression with a bound parameter.
[ticket:2744]

Conflicts:
	doc/build/changelog/changelog_09.rst