[AIRFLOW-809] Use __eq__ ColumnOperator When Testing Booleans
[AIRFLOW-809] Use __eq__ ColumnOperator When Testing Booleans

The .is_ ColumnOperator causes the SqlAlchemy's MSSQL dialect to produce
IS 0 when given a value of False rather than a value of None. The __eq__
ColumnOperator does this same test with the added benefit that it will
modify the resulting expression from and == to a IS NULL when the target
is None.

This change replaces all is_ ColumnOperators that are doing boolean
comparisons and leaves all is_ ColumnOperators that are checking for
None values.

gritlogic/incubator-airflow,gritlogic/incubator-airflow,gritlogic/incubator-airflow,gritlogic/incubator-airflow