Support multiValueHeaders on Response

[koxudaxi]

@jordaneremieff
I suggest to support multiValueHeaders on Response.
I want to set the same-name cookies.
FastAPI + uvicorn can return same-name cookies.
But, Mangum returned only the last cookie. the first cookie is dropped.
Mangum should support multiValueHeaders to resolve the problem.

This change affects a little bit of performance.
If You prefer this PR, then I will add test the code.

How do you think about it?

https://docs.aws.amazon.com/apigateway/latest/developerguide/set-up-lambda-proxy-integrations.html#api-gateway-simple-proxy-for-lambda-output-format

example

from fastapi import FastAPI
from starlette.responses import Response
from mangum import Mangum
app = FastAPI()

@app.get('/')
def get_root():
    response = Response()
    response.set_cookie('test', 'value1')
    response.set_cookie('test', 'value2')
    return response


if __name__ == '__main__':
    import uvicorn
    uvicorn.run(app)
else:
    handler = Mangum(app)

output

curl -v http://127.0.0.1:8000/
*   Trying 127.0.0.1...
* TCP_NODELAY set
* Connected to 127.0.0.1 (127.0.0.1) port 8000 (#0)
> GET / HTTP/1.1
> Host: 127.0.0.1:8000
> User-Agent: curl/7.58.0
> Accept: */*
> 
< HTTP/1.1 200 OK
< date: Tue, 23 Jun 2020 15:19:09 GMT
< server: uvicorn
< set-cookie: test=value1; Path=/; SameSite=lax
< set-cookie: test=value2; Path=/; SameSite=lax
< transfer-encoding: chunked
< 
* Connection #0 to host 127.0.0.1 left intact

[jordaneremieff]

@koxudaxi Looks like a good addition, I will merge this when it is ready. Thanks!

[koxudaxi]

@jordaneremieff
I have added unit tests.

By the way, Why CI does not run for this PR?

[jordaneremieff]

@koxudaxi thanks. Not sure why it doesn't appear on Github, but the build does run successfully in CI https://travis-ci.org/github/erm/mangum/builds/701506590. I'll try to review this soon then will merge.