Add header accessors

[ahopkins]

This PR adds a single accessor to get all headers in a concatenated string of all values.

Example-Field: Foo, Bar
Example-Field: Baz

request.headers.example_field == "Foo, Bar,Baz"

Accessors will translate _ to -, and will ignore a trailing _.

[codecov]

Codecov Report

Base: 88.720% // Head: 88.720% // No change to project coverage 👍

Coverage data is based on head (a927f45) compared to base (dfc0704).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #2696   +/-   ##
=========================================
  Coverage   88.720%   88.720%           
=========================================
  Files           87        87           
  Lines         6853      6853           
  Branches      1177      1177           
=========================================
  Hits          6080      6080           
  Misses         532       532           
  Partials       241       241           

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.