fix: don't panic on missing headers under SAM

[Fishrock123]

Description of changes:

SAM is missing these headers, making local testing impossible without this change.

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[coltonweaver]

This is not the first time we've received this request, and I understand the issue you're facing. However, I don't believe that this runtime is the right place to inject these default values, and could lead to weird behavior for developers relying on this in production Lambda.

#314

I would recommend looking at the comment made by @davidbarsky on this previous PR and reach out to the upstream repo to try and fix this regression. aws/aws-sam-cli#2715

[coltonweaver]

I'm going to close this PR as I don't believe this runtime to be the right place to try and remediate this issue. If you strongly disagree, please feel free to reply and we can discuss.

[Fishrock123]

I feel like it should be vocal about these missing somehow but I don't know that it is worth crashing? Not sure.