Fix --user-agent='' making k6 to not send user agent

[mstoykov]

No description provided.

[codecov-io]

Codecov Report

Merging #1695 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1695      +/-   ##
==========================================
+ Coverage   72.33%   72.37%   +0.04%     
==========================================
  Files         176      176              
  Lines       13536    13536              
==========================================
+ Hits         9791     9797       +6     
+ Misses       3132     3126       -6     
  Partials      613      613              

Flag	Coverage Δ
#ubuntu	72.33% <100.00%> (+0.01%)	⬆️
#windows	70.86% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
js/modules/k6/http/request.go	80.71% <100.00%> (ø)
stats/cloud/collector.go	79.81% <0.00%> (-0.64%)	⬇️
lib/executor/ramping_arrival_rate.go	97.42% <0.00%> (+1.03%)	⬆️
core/engine.go	92.96% <0.00%> (+2.01%)	⬆️
lib/testutils/minirunner/minirunner.go	86.04% <0.00%> (+4.65%)	⬆️

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