fix: should panic when define duplicate field param in required_if

[duyquang6]

Fixes Or Enhances

fix #1466

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• Tests exist or have been written that cover this particular change.

@go-playground/validator-maintainers

[coveralls]

coverage: 73.684% (+0.007%) from 73.677%
when pulling 7458d8a on duyquang6:fix/should-panic-required-if-duplicate-fieldname
into 6905468 on go-playground:master.

[nodivbyzero]

Looks good to me. Just a couple of follow-ups:

1. We should probably add the same logic to required_unless.
2. Please update doc.go to reflect the new behavior.

[duyquang6]

@nodivbyzero

• Updated doc.go
• required_unless is a little bit different, I found that it work fine with duplicate params, so I just add testcase for it