Fix Postgres array syntax for UPDATE

[raxod502]

This PR should address #579 by formatting UPDATE queries using the same rules as INSERT queries. Regression tests for the new case have been added, courtesy of @vfonte91.

A side effect of this fix is that I discovered PyPika has inconsistent handling of null versus NULL. In these queries, null is used:

SELECT * FROM "abc" WHERE "foo"=null
UPDATE "abc" SET "foo"=null

But in these queries, NULL is used:

INSERT INTO "abc" VALUES (NULL)
INSERT INTO "abc" VALUES (1) RETURNING NULL
SELECT NULL "abcdef"

This is because there are two different code paths for formatting null values:

pypika/pypika/terms.py

Line 385 in a7b01da

return "null"

pypika/pypika/terms.py

Line 479 in a7b01da

super().__init__("NULL", alias)

Both of these code paths used to return lowercase null circa 2018; this was changed in a seemingly unrelated commit 3bfee98.

Some other keywords (true, false) seem to be lowercase by convention in PyPika, whereas others (SYSTEM_TIME) seem to be uppercase by convention, so it's not entirely clear to me what the expected standardization is. On the plus side, since SQL is fully case-insensitive, it doesn't really matter too much.

In this PR, I've updated all code paths and tests to use lowercase null. Let me know if that's wrong.

[levintrix]

would love this to be merged :(

[foxx]

@ Maintainers - any chance of getting this merged?