update on conflict with condition

[zidaye]

close #734
refer to：
https://www.postgresql.org/docs/current/sql-insert.html

[baoyachi]

LGTM

[alamb]

Thank you @zidaye

[alamb]

My preference is to call this where to match the SQL more closely -- I will make this change in a follow on PR

[alamb]

Actually, now I see the name selection is consistent with the rest of this crate (e.g.

sqlparser-rs/src/ast/mod.rs

Line 1139 in faf75b7

selection: Option<Expr>,

)

I will not make any other changes

[coveralls]

Pull Request Test Coverage Report for Build 3573700071

• 46 of 48 (95.83%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 86.378%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/ast/mod.rs	8	10	80.0%

Totals
Change from base Build 3568725328:	0.02%
Covered Lines:	11921
Relevant Lines:	13801

---

💛 - Coveralls

[alamb]

Thanks again @zidaye and @baoyachi