fix(dao): HSTORE checking like ENUM

This allows user can define HSTORE attribute directly.
fix(hstore): check string before parsing
fix(dialects): replace HSTORE checking fn with toString
test(HSTORE): add some related tests
Merge pull request #1458 from tomchentw/hotfix/hstore_checking_fn

fix(dao): HSTORE checking like ENUM
Update changelog.md