Write response headers only once

[vigneshshanmugam]

• write the response headers only one
• Fire the response event only after extracting the preload headers.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@c93b659). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff            @@
##             master    #145   +/-   ##
========================================
  Coverage          ?   92.3%           
========================================
  Files             ?      13           
  Lines             ?     507           
  Branches          ?      89           
========================================
  Hits              ?     468           
  Misses            ?      39           
  Partials          ?       0

Impacted Files	Coverage Δ
lib/request-handler.js	96.96% <100%> (ø)

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