Deparse SELECT NULL and BooleanTest.

[jcsjcs]

These changes to the deparser handle:
SELECT NULL
WHERE a IS true
WHERE a is false
WHERE a IS NOT true
WHERE a IS NOT false

[lfittl]

Given that https://www.postgresql.org/docs/9.5/static/functions-comparison.html has true/false uppercase (i.e. TRUE/FALSE), I'd be more inclined to deparse to uppercase as well.

[jcsjcs]

Makes sense. I'll correct.

[jcsjcs]

Looking at https://www.postgresql.org/docs/9.5/static/functions-comparison.html, I think I should add the deparsing of IS UNKNOWN and IS NOT UNKNOWN at well - for completeness sake.

[lfittl]

@jcsjcs Good point - do you want to add that to this PR as well?

[lfittl]

Sorry for one more nitpick - since this variable never changes, we could move it on top of the method and freeze it, to save an allocation, like this:

BOOLEAN_TEST_TYPE_TO_STRING = {
  BOOLEAN_TEST_TRUE        => ' IS TRUE',
  BOOLEAN_TEST_NOT_TRUE    => ' IS NOT TRUE',
  BOOLEAN_TEST_FALSE       => ' IS FALSE',
  BOOLEAN_TEST_NOT_FALSE   => ' IS NOT FALSE',
  BOOLEAN_TEST_UNKNOWN     => ' IS UNKNOWN',
  BOOLEAN_TEST_NOT_UNKNOWN => ' IS NOT UNKNOWN'
}.freeze
def deparse_boolean_test(node)
  deparse_item(node['arg']) + BOOLEAN_TEST_TYPE_TO_STRING[node['booltesttype']]
end

[jcsjcs]

No problem, done.