Fix genUpdateColumns() should not ignore soft delete column when unscoped

[elvinchan]

For example,

type Object struct {
	Id        int64
	DeletedAt time.Time `xorm:"deleted"`
}

Now I want to restore a deleted record, if I use

engine.ID(1).Unscoped().Nullable("deleted_at").Update(&Object{})

It's OK, but if I use

engine.ID(1).Unscoped().Cols("deleted_at").Update(&Object{})

It has no effect.

I think the effect of these two way should be consistent.

[codecov-io]

Codecov Report

Merging #1051 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1051      +/-   ##
==========================================
+ Coverage   54.27%   54.31%   +0.03%     
==========================================
  Files          41       41              
  Lines        7551     7551              
==========================================
+ Hits         4098     4101       +3     
+ Misses       2936     2935       -1     
+ Partials      517      515       -2

Impacted Files	Coverage Δ
session_update.go	52.54% <100%> (+0.63%)	⬆️
xorm.go	66.15% <0%> (+1.53%)	⬆️

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[lunny]

Thanks!