support User-Agent change and deletion from -H, simplify UI for headers

[ldemailly]

support User-Agent change and deletion from -H, simplify UI for headers to be extra headers only.

fixes #648

[codecov]

Codecov Report

Base: 88.4% // Head: 88.4% // Increases project coverage by +0.0% 🎉

Coverage data is based on head (7901945) compared to base (224f9c7).
Patch coverage: 89.5% of modified lines in pull request are covered.

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #649   +/-   ##
======================================
  Coverage    88.4%   88.4%           
======================================
  Files          30      30           
  Lines        4350    4361   +11     
======================================
+ Hits         3847    3857   +10     
- Misses        337     338    +1     
  Partials      166     166           

Impacted Files	Coverage Δ
rapi/restHandler.go	87.0% <ø> (-<0.1%)	⬇️
fhttp/http_client.go	84.3% <85.7%> (-0.1%)	⬇️
fhttp/http_forwarder.go	90.3% <100.0%> (ø)
fhttp/http_utils.go	95.6% <100.0%> (+<0.1%)	⬆️
dflag/configmap/updater.go	80.8% <0.0%> (-1.0%)	⬇️
fnet/network.go	90.1% <0.0%> (+0.5%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.

[wiardvanrij]

Implementation looks good, funny how something small ends up into this :)

The only nit I have, is that technically if you would enforce a linter on it (comments), it's going to say something about the style. It should start with a capital and end with a dot .. But I'll leave it up to you if you want to start making that the default/standard to make the comments more uniform.

[ldemailly]

While I totally agree it is useful for documentation to have proper complete sentences, I dont think making complete sentences of local comments in test files or middle of code is useful. even cosmetically I think it detracts from their purpose (and more importantly puts incentive to not comment)