fix/ensure array params are sent to the server correctly

[awalker4]

Fixes #135

The SDK sends the wrong formdata when the user adds an array param. For instance, the following call:

client.general.partition({
    partitionParameters: {
        files: {
            content: data,
            fileName: filename
        },
        extractImageBlockTypes: ["Image", "Table"],
    }
})

Sends the list as one comma separated FormData value: `extract_image_block_types: "Image,Table"'.

The server will ignore this unless it's sent with multiple keys like so: extract_image_block_types[]: "Image", extract_image_block_types[]: "Table"

We addressed this before in #122 by adjusting the request body before sending it, but this code path is for pdf splitting. A better solution is to add a new hook that will clean up the form data for every request. Then we can remove the modification in the splitting code.