fix: panic whith custom http headers

[caarlos0]

closes #145

[codecov]

Codecov Report

Merging #149 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #149   +/-   ##
=======================================
  Coverage   83.27%   83.27%           
=======================================
  Files          20       20           
  Lines        1160     1160           
=======================================
  Hits          966      966           
  Misses        174      174           
  Partials       20       20           

Impacted Files	Coverage Δ
htmltest/options.go	90.41% <100.00%> (ø)

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