[http-multipart-body-parser] Parse array from multipart form data

[benjifs]

What does this implement/fix? Explain your changes.

When receiving formdata with an array (eg. checkboxes), only the last value is used. A simple way to test is by trying out the following:

curl -X POST -F 'foo[]=one' -F 'foo[]=two' http://localhost

Currently this would parse as

'foo[]': 'two'

With this change, the result is:

'foo': [ 'one', 'two' ]

Does this close any currently open issues?

No

Any relevant logs, error output, etc?

n/a

Any other comments?

I originally created #508 but rebasing was causing some issues on my end. Made a clean branch and put the changes here instead.

Where has this been tested?

Node.js Versions: 13.12.0

Middy Versions: 1.0.0

AWS SDK Versions: n/a

Todo list

[x] Feature/Fix fully implemented
[x] Added tests
[ ] Updated relevant documentation
[ ] Updated relevant examples