fix fixed An error occurred when only one file was uploaded in batches and add unit testing(#2092)

[tiger1103]

修复规范路由下批量上传时，若只上传一个文件出现错误：reflect.Value.Convert: value of type *ghttp.UploadFile cannot be converted to type ghttp.UploadFile
修复前单测：

修复后：

[codecov-commenter]

Codecov Report

Base: 79.55% // Head: 79.61% // Increases project coverage by +0.05% 🎉

Coverage data is based on head (a7f7259) compared to base (b0c9c68).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2357      +/-   ##
==========================================
+ Coverage   79.55%   79.61%   +0.05%     
==========================================
  Files         589      589              
  Lines       48803    48803              
==========================================
+ Hits        38825    38853      +28     
+ Misses       8004     7984      -20     
+ Partials     1974     1966       -8     

Flag	Coverage Δ
go-1.15-386	79.60% <100.00%> (+<0.01%)	⬆️
go-1.15-amd64	79.64% <100.00%> (+0.09%)	⬆️
go-1.16-386	79.63% <100.00%> (+0.03%)	⬆️
go-1.16-amd64	79.65% <100.00%> (+0.09%)	⬆️
go-1.17-386	79.61% <100.00%> (+0.02%)	⬆️
go-1.17-amd64	79.63% <100.00%> (+0.05%)	⬆️
go-1.18-386	79.58% <100.00%> (+0.06%)	⬆️
go-1.18-amd64	79.54% <100.00%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
net/ghttp/ghttp_request_param_request.go	90.16% <100.00%> (+2.45%)	⬆️
os/glog/glog_logger_rotate.go	64.70% <0.00%> (+1.35%)	⬆️
util/gconv/gconv_struct.go	81.01% <0.00%> (+3.24%)	⬆️
os/gfsnotify/gfsnotify_watcher_loop.go	84.87% <0.00%> (+6.72%)	⬆️

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[gqcn]

@tiger1103 你好，感谢贡献！不过这里的问题主要出在转换组件那里，因此我新开了一个分支，合并了你的代码，并做了些改动提交了新的PR：#2365