Fix batch INSERT when using ON CONFLICT

[bretep]

Fixes issue #505

[coveralls]

Pull Request Test Coverage Report for Build 97

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 72.319%

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Totals
Change from base Build 94:	0.04%
Covered Lines:	1126
Relevant Lines:	1557

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 97

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 72.319%

---

Totals
Change from base Build 94:	0.04%
Covered Lines:	1126
Relevant Lines:	1557

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 97

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 72.319%

---

Totals
Change from base Build 94:	0.04%
Covered Lines:	1126
Relevant Lines:	1557

---

💛 - Coveralls

[bretep]

The failure is unrelated to this commit and fails only for go1.8

[bretep]

@hmgle are you able to merge this?

[bretep]

I've been using this patch for the past 20 or so days. It's been working as expected. Thanks @hmgle

[hmgle]

@jmoiron Could you please review and handle this pr? Thanks.

[bretep]

Thanks @hmgle for pingin @jmoiron

@jmoiron Hopefully we can get this committed soon so that I don't have to maintain a fork. :) Please let me know if I need to take any additional steps to get this PR accepted.

[jmoiron]

The 1.8 failure is due to the mysql driver dropping support for 1.8; I believe my backwards compatability guarantee also drops 1.8 so I'll remove it from the testing matrix instead of trying to fix it.

Additionally, since there has been no tagged release yet for this feature (I want to get more time to live with it), I don't mind removing an exported identifier... in fact, I prefer it, it wasn't necessary to export in the first place.

[bretep]

Thank you!