Adding setter to should_evaluate_none property

Adding setter to should_evaluate_none property

Fixed issue where the :class:`.JSON` type had a read-only
:attr:`.JSON.should_evaluate_none` attribute, which would cause failures
when making use of the :meth:`.TypeEngine.evaluates_none` method in
conjunction with this type.  Pull request courtesy Sanjana S.

Fixes: #4485
Closes: #4496
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4496
Pull-request-sha: 044beb23982d411be6fe640716b1b693df0f7189

Change-Id: I1f3e1d7dec9d2ceb6ccaaa8cac158a062cf02710