fix http: invalid Content-Length of "-1"

[evildao]

When the message is empty, Content-Length returns -1, causing the http library to throw an error log

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.62%. Comparing base (ef7d482) to head (dd9de7b).
Report is 40 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #97      +/-   ##
==========================================
- Coverage   81.35%   76.62%   -4.74%     
==========================================
  Files           3        3              
  Lines         118      154      +36     
==========================================
+ Hits           96      118      +22     
- Misses         19       32      +13     
- Partials        3        4       +1     

Flag	Coverage Δ
go-	?
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	76.62% <100.00%> (-4.74%)	⬇️
go-1.22	76.62% <100.00%> (-4.74%)	⬇️
go-1.23	76.62% <100.00%> (?)
macos-latest	?
ubuntu-latest	76.62% <100.00%> (-4.74%)	⬇️

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