Before 34cc301f, type casting by boolean attribute when querying is a
no-op, so finding by truthy boolean string (i.e.
`where(value: "true") # => value = 'true'`) didn't work as expected
(matches it to FALSE in MySQL #32624). By type casting is ensured, a
value on boolean attribute is always serialized to TRUE or FALSE.

In PostgreSQL, `where(value: :false) # => value = 'false'` was a valid
SQL, so 34cc301f is a regresson for PostgreSQL since all symbol values
are serialized as TRUE.

I'd say using `:false` is mostly a developer's mistake (user's input
basically comes as a string), but `:false` on boolean attribute is
serialized as TRUE is not a desirable behavior for anybody.

This allows falsy boolean symbols as false, i.e.
`klass.create(value: :false).value? # => false` and
`where(value: :false) # => value = FALSE`.

Fixes #35676.