Add multi-value headers to the API Gateway response

[piotrkubisa]

I noticed multiValueHeaders is missing in the API Gateway response event.

• Relates to Add support for multi-value parameters in API Gateway proxy events #136.
• https://docs.aws.amazon.com/apigateway/latest/developerguide/set-up-lambda-proxy-integrations.html#api-gateway-simple-proxy-for-lambda-output-format

[codecov-io]

Codecov Report

Merging #155 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #155   +/-   ##
=======================================
  Coverage   68.89%   68.89%           
=======================================
  Files          17       17           
  Lines         659      659           
=======================================
  Hits          454      454           
  Misses        165      165           
  Partials       40       40

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[bmoffatt]

Thanks!