fix: create a valid URL for REST APIs and ALBs when the host header i…

[nmoutschen]

…s missing

Issue #, if available: #368

Description of changes:

When the Host header is missing from an API Gateway REST API or ALB request, the current implementations defaults to an empty value. This means that the lambda_http crate tries to transform a value of https:///my/path into an Uri, which results in an http::Error(InvalidUri(InvalidFormat)) error.

This PR changes the behavior by checking first if there is a valid host header present. It then uses the http(s)://my-host/my/path pattern if it is present, or just /my/path otherwise.

No changes are needed for API Gateway v2 since it adds a requestContext.domainName value, which means we can always derive a valid domain name in that situation.

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[coltonweaver]

LGTM, thanks for fixing!