add test to return multiple headers when the multi-value headers opton is enabled in ALB

[jurasofish]

mangum splits the header response between the headers and multiValueHeaders items.

When the multi-value headers option is enabled all headers must be under multiValueHeaders, as described here https://docs.aws.amazon.com/elasticloadbalancing/latest/application/lambda-functions.html#multi-value-headers.

What mangum produces:

'headers': {'content-type': 'text/plain; charset=utf-8'}, 'multiValueHeaders': {'set-cookie': ['cookie1=cookie1; Secure', 'cookie2=cookie2; Secure']}

What should be produced:

'multiValueHeaders': {'content-type': ['text/plain; charset=utf-8'], 'set-cookie': ['cookie1=cookie1; Secure', 'cookie2=cookie2; Secure']}

[jordaneremieff]

Hi @jurasofish, are there changes in your draft PR needed to run these tests? Please let me know the statuses and I can review/merge.

[jurasofish]

Hi @jurasofish, are there changes in your draft PR needed to run these tests? Please let me know the statuses and I can review/merge.

hey, yes, #189 sorts everything out