create drop schema conditionally

[alex-dukhno]

added CREATE SCHEMA IF NOT EXISTS ... for PostgreSQL
added test for DROP SCHEMA IF EXISTS ... as it was supported but no test for the case

[coveralls]

Pull Request Test Coverage Report for Build 283916036

• 22 of 26 (84.62%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.04%) to 91.83%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/parser.rs	3	4	75.0%
src/ast/mod.rs	4	7	57.14%

Totals
Change from base Build 279659934:	-0.04%
Covered Lines:	4687
Relevant Lines:	5104

---

💛 - Coveralls

[nickolay]

Thanks! See my two comments below.

This continues from #173 and I'll copy the spec links from that PR: SQL:2016 and Posgres. The IF NOT EXISTS is a Postgres-specific clause.

[nickolay]

Using "CREATE SCHEMA {if_not_exists}{name}" and an empty string for if_not_exists when it needs to be omitted both is consistent with other implementations, and results in more compact code.

[nickolay]

Please default to pg_and_generic() unless testing a construct that can't be parsed by the generic parser. The idea of the generic parser is to be able to parse as much SQL code as possible.