Add default to http_serde::header_map::deserialize

[dacut]

The Go SDK typically has omitEmpty for Rust's http::HeaderMap entries, and services like ALB omit them when they're not present. However, without a default parameter to the #[serde()] annotation, deserialization fails when they're missing instead of the desired behavior of producing an empty HeaderMap.

This changes #[serde(deserialize_with = "http_serde::header_map::deserialize")] annotations to #[serde(deserialize_with = "http_serde::header_map::deserialize", default)] so we get the desired behavior. A test is also added (in aws_lambda_events/src/custom_serde.rs -- feel free to suggest a better place if there is one) to verify the correctness of this change.

[dacut]

This has been verified in my own Lambda function behind an ALB, FWIW.

[LegNeato]

Thank you for the PR!