Pass Lambda Context to web app in a new header

[mbfreder]

Issue #, if available: #244

Description of changes:

Made Lambda Context available to web apps in a new header (x-amzn-lambda-context).

Note : The implementation broke some tests because the LambdaRequest object used to send the request to the mock server doesn't have a lambda context, resulting in an error ("Request did not contain a lambda context") when "let lambda_context = &event.lambda_context();" is called. I fixed this by replacing LambdaRequest with Request, which allows adding the lambda context.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[bnusunny]

Thanks for the contribution.

Could you add one test to check if the new header is present? You can do it when creating the mock server.

        when.method(GET)
            .path("/")
            .header_exists("x-amzn-lambda-context");

[mbfreder]

Thanks for the contribution.

Could you add one test to check if the new header is present? You can do it when creating the mock server.

        when.method(GET)
            .path("/")
            .header_exists("x-amzn-lambda-context");

Done, thanks.