Support Request Level Custom Http Headers..

[whybeyoung]

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Issue: I dont' find a request level method to set header for a request. this is needed in some cases

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 98.52%. Comparing base (774fc9d) to head (dc6bca5).
Report is 75 commits behind head on master.

Files with missing lines	Patch %	Lines
client.go	40.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #909      +/-   ##
==========================================
+ Coverage   98.46%   98.52%   +0.06%     
==========================================
  Files          24       26       +2     
  Lines        1364     1767     +403     
==========================================
+ Hits         1343     1741     +398     
- Misses         15       18       +3     
- Partials        6        8       +2     

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