Don't assume that Hstore columns have always changed

HStore columns come back from the database separated by a comma and a
space, not just a comma. We need to mirror that behavior since we
compare the two values.

Also adds a regression test against JSON to ensure we don't have the
same bug there.
Merge pull request #15780 from sgrif/sg-dont-always-save-mutable-types

Don't assume that Hstore columns have always changed
[ci skip] fix doc for the Hash#assert_valid_keys
Fix documentation from #15669 [ci skip]
Don't type cast the default on the column

If we want to have type decorators mess with the attribute, but not the
column, we need to stop type casting on the column. Where possible, we
changed the tests to test the value of `column_defaults`, which is
public API. `Column#default` is not.