Normalise headers dictionary keys: fixes issues 1860, 3083 etc.

[mastazi]

Which issue(s) does this change fix?

fixes #1860, #3083 etc.

Why is this change necessary?

In API Gateway HTTP API event all headers are stored in the headers dictionary always lowercase, for example these raw headers:

Foo: baz
bar: baz

note how one header name is capitalised (Foo) and the other isn't (bar).

In API Gateway, this will result in

"event": {
  "headers": {
    "foo": "baz",
    "bar": "baz"
  }
}

note how foo and bar are both lowercase.

SAM will also treat headers as case-insensitive but unfortunately the case is capitalised instead of lowercase:

"event": {
  "headers": {
    "Foo": "baz",
    "Bar": "baz"
  }
}

this PR makes SAM behave like API Gateway.

How does it address the issue?

By making the keys of the headers dictionary lowercase, as they are in API Gateway.

What side effects does this change have?

users who were dealing with this inconsistency as in the example below, will have to modify their code:

if (process.env.AWS_SAM_LOCAL) {
    myvar  = event.headers.Myheader;
} else {
    myvar = event.headers.myheader;
}

because now myheader will be found both in SAM Local and in API Gateway. So the if-else condition is not required.

Checklist

• Add input/output type hints to new functions/methods
• Write design document (Do I need to write a design document?)
• Write unit tests
• Write/update functional tests
• Write/update integration tests
• make pr passes
• make update-reproducible-reqs if dependencies were changed
• Write documentation

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[mastazi]

Please allow me 1-2 days I will fix tests failing.
I would put the PR as draft but for some reason I don't see the button to convert to draft.

[mastazi]

Unfortunately my local linter settings polluted this PR. I am closing it and will reopen a new one.