add Content-Type header

[nanmu42]

Not mandatory, but good for Internet.

[codecov]

Codecov Report

Merging #2 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #2      +/-   ##
==========================================
+ Coverage   78.34%   78.43%   +0.08%     
==========================================
  Files          10       10              
  Lines         254      255       +1     
==========================================
+ Hits          199      200       +1     
  Misses         33       33              
  Partials       22       22

Impacted Files	Coverage Δ
client.go	52.11% <100%> (+0.68%)	⬆️

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