Fix gdb Order

[FlyingBlazer]

if model.orderBy != "" {
    model.orderBy += ","
}

既然前边已经有了这样的判断，说明Order方法本意是允许多次调用.OrderDesc("a").OrderDesc("b")来生成类似于order by a desc, b desc这样的查询语句的，所以这里应该使用+=而不是=

[codecov-commenter]

Codecov Report

Merging #1520 (47cefbf) into master (99455e3) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1520      +/-   ##
==========================================
- Coverage   69.79%   69.78%   -0.01%     
==========================================
  Files         438      438              
  Lines       42421    42421              
==========================================
- Hits        29609    29605       -4     
- Misses      10917    10920       +3     
- Partials     1895     1896       +1     

Flag	Coverage Δ
go-1.15	69.75% <100.00%> (+0.03%)	⬆️
go-1.16	69.75% <100.00%> (-0.03%)	⬇️
go-1.17	69.72% <100.00%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
database/gdb/gdb_model_order_group.go	47.05% <100.00%> (-8.83%)	⬇️
os/glog/glog_logger_rotate.go	65.85% <0.00%> (-1.47%)	⬇️
net/gtcp/gtcp_conn.go	31.15% <0.00%> (-0.51%)	⬇️
os/gmutex/gmutex.go	97.36% <0.00%> (+2.63%)	⬆️

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