fix: add support for null/invalid type for headers

[nmoutschen]

Issue #, if available: #367

Description of changes:

This adds support for invalid/null value for headers, defaulting to an empty HeaderMap if need be, increasing resilience if the Lambda function would get an invalid value for headers.

I've also added additional checks in the request deserialization tests to ensure we deserialize from the right type of API, as making deserialization more permissive could potentially break this.

❓ Open question before merging this: this change is overly permissive. Should I scope this down to only accepting null and map values?

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[coltonweaver]

LGTM! Thanks for doing this.

WRT your overly permissive question - I think it's fine to do things like this in lambda_http.