Feat: Support multipart form data with same attribute name

[quangpdt]

When uploading file with multipart/form-data, there's ability to send the list of value, with same attribute name.

Let's have a look on this payload, assuming we're uploading a document with some additional information, these information will be attached into multipart/form-data. Here's an example of the payload pass from webview to the plugin:

{
    foo: 'First information',
    bar: 'Second information'
}

What if user want to send a list of bar, instead only 1 string value?

{
    foo: 'First information',
    bar: ['Second information', 'Third information']
}

I added the ability to check the type of the value, before adding it to the form data. If it's a string, we'll go ahead as usual. If it's a string array, each item will be added to the form data, with a same name.

See more example of JS implementation at: https://developer.mozilla.org/en-US/docs/Web/API/FormData/append

[quangpdt]

Hi @thomasvidas , could you please give feedbacks on my PR? It's just a small one and won't take much time to review.

Thank you!