fix: calculate accurate body size when posting files

[Kybxd]

• close stats: set stats.BodySize from multipart writer #41

[codecov]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 81.06%. Comparing base (a513960) to head (f4278ea).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
client.go	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
+ Coverage   80.48%   81.06%   +0.58%     
==========================================
  Files           6        6              
  Lines         415      412       -3     
==========================================
  Hits          334      334              
+ Misses         56       54       -2     
+ Partials       25       24       -1     

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[wenchy]

We'd better add test case to compare the wanted size.