Clarify the format of request.headers

[dobesv]

I spent a few minutes trying to figure out what format the headers were being provided in, perhaps this will save some time for anyone else (or my future self) who forgets the details of how header names are lower-cased and how multiple headers with the same name are combined, etc..

[codecov]

Codecov Report

Merging #1325 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1325   +/-   ##
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  Coverage     100%    100%           
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  Files           5       5           
  Lines         393     393           
======================================
  Hits          393     393

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