去除 version 字段的 Add 条件

[ghost]

当 结构体开启 version tag 时, 使用 update 方法, 第一个参数(更新的字段)结构体指针中 Version 字段默认为 0, 导致找不到更新的数据.
调用的方式为:

update := &FeedItem{
	DatePublished: "2019-01-9",
}
engine.Where("date_published = ?", "2019-01-11").Update(update)

生成的 sql 类似于:

[xorm] [info]  2019/02/14 17:36:46.057228 [SQL] UPDATE "feed_item" SET "date_published" = $1, "updated" = $2, "version" = "version" + 1 WHERE (date_published = $3) AND "version"=$4 []interface {}{"2019-01-9", 1550137006, "2019-01-11", 0}

[codecov-io]

Codecov Report

Merging #1230 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1230      +/-   ##
==========================================
- Coverage   57.35%   57.33%   -0.02%     
==========================================
  Files          43       43              
  Lines        7797     7796       -1     
==========================================
- Hits         4472     4470       -2     
  Misses       2771     2771              
- Partials      554      555       +1

Impacted Files	Coverage Δ
session_update.go	55.55% <ø> (-0.15%)	⬇️
xorm.go	68.18% <0%> (-1.52%)	⬇️

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[lunny]

Could you add some tests?