Content-type header is not lowercased

[nponeccop]

Bug Report

It's a difference in behavior between serverless-offline and real AWS APIGateway V2 aka events.httpApi.

Expected Behavior

event.headers.content-type property is lowercased even if it's passed by curl as cOnteNt-type: , because AWS lowercases it.

Current Behavior

event.headers.content-type arrives the same way an HTTP client passed it. E.g. curl --no-progress-meter -X POST -d '{ "code": 2}' -H 'Content-type: application/json' would be capitalized in the event object.

By contrast, AWS always normalizes it to lowercase content-type.

I'm not aware whether http or headers other than content-type are affected too

Sample Code

It works in the aws-nodejs template, with http replaced by httpApi. I can provide with a repo should a need arise, but it's really simple.