rpc (fix): Set missing Accept header for RPC requests

[xerial]

Fixes #3083

[codecov]

Codecov Report

Merging #3087 (3e431d0) into master (ed1faeb) will increase coverage by 0.02%.
The diff coverage is 85.71%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3087      +/-   ##
==========================================
+ Coverage   82.84%   82.86%   +0.02%     
==========================================
  Files         349      349              
  Lines       14675    14681       +6     
  Branches     2498     2487      -11     
==========================================
+ Hits        12157    12165       +8     
+ Misses       2518     2516       -2     

Files Changed	Coverage Δ
.../wvlet/airframe/http/client/HttpClientConfig.scala	53.33% <50.00%> (-0.24%)	⬇️
...wvlet/airframe/http/client/HttpClientFactory.scala	60.00% <100.00%> (+22.50%)	⬆️
...scala/wvlet/airframe/http/client/HttpClients.scala	89.07% <100.00%> (+0.09%)	⬆️

... and 3 files with indirect coverage changes

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