fix: operator.update, columns in where cant be updated

[sunfuze]

#26

@dead-horse

[mention-bot]

@sunfuze, thanks for your PR! By analyzing the history of the files in this pull request, we identified @fengmk2, @dead-horse and @csbun to be potential reviewers.

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@7cb4e75). Click here to learn what that means. The diff coverage is 100%.

@@            Coverage Diff            @@
##             master      #27   +/-   ##
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  Coverage          ?   98.92%           
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  Files             ?        6           
  Lines             ?      278           
  Branches          ?       40           
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  Hits              ?      275           
  Misses            ?        3           
  Partials          ?        0

Impacted Files	Coverage Δ
lib/operator.js	100% <100%> (ø)

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[sunfuze]

db.update('table', { field1: 'abc', field2: 'efg' }, {
  where: { field1: 'a', id: 1 }
})`

原来上面这种情况，是不会更新 field1的。

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db.update('table', { id: 1, field1: 'abc' })

上面的代码生成的 sql 不应该有 'SET id = 1' 的内容。