format time when sqlTypeName is core.Varchar

[DarthPestilane]

I believe that many developers define the created_at or updated_at columns as a string like this:

type User struct {
    Id int64
    // ... other cols
    CreatedAt string `xorm:"'created_at', created"`
}

While using postgres database, a date time string with decimal, like 2018-06-28 15:33:58.942773, will be inserted. But in fact, we don't want a time(string) with decimal.
So I add some codes to format the time value into 2006-01-02 15:04:05:

// ... context
case core.DateTime, core.TimeStamp, core.Varchar: // !DarthPestilane! format time when sqlTypeName is core.Varchar.
    v = t.Format("2006-01-02 15:04:05")
// ... context

Hope can be merged.

[codecov-io]

Codecov Report

Merging #1026 into master will decrease coverage by 3.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1026      +/-   ##
==========================================
- Coverage   57.40%   54.30%   -3.11%     
==========================================
  Files          44       41       -3     
  Lines        7912     7552     -360     
==========================================
- Hits         4542     4101     -441     
- Misses       2806     2934     +128     
+ Partials      564      517      -47     

Impacted Files	Coverage Δ
engine.go	61.51% <100.00%> (-1.09%)	⬇️
dialect_mssql.go	0.00% <0.00%> (-73.81%)	⬇️
session_exist.go	57.44% <0.00%> (-5.52%)	⬇️
rows.go	50.76% <0.00%> (-5.38%)	⬇️
session_update.go	51.91% <0.00%> (-4.71%)	⬇️
helpers.go	57.41% <0.00%> (-2.32%)	⬇️
tag.go	77.09% <0.00%> (-2.22%)	⬇️
session.go	56.29% <0.00%> (-2.20%)	⬇️
session_cols.go	91.30% <0.00%> (-2.18%)	⬇️
xorm.go	64.61% <0.00%> (-2.06%)	⬇️
... and 24 more

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[lunny]

@DarthPestilane could you add some tests?