Handling dynamic headers from transformFile

Hi there!

We have a situation where we use rc-upload in the following manner:

• User selects file on file system
• We use transformFile to encrypt the contents of that file on the browser
• The encryption generates additional metadata that we want to pass as headers
• This metadata is not available until transformFile is completed.

It would be great if headers could also be a function that takes in the result of transformFile and returns the headers.

I tried to make a PR for this but it was not possible bc of permissions.

The changes to do this are pretty quick:
in AjaxUploader.tsx

change

headers: props.headers,

to

headers: typeof props.headers === 'function' ? props.headers(transformedFile) : props.headers,

and also update the README comment to me

|headers| object/function(blob) | {} | http headers to post, available in modern browsers. Can be a function that takes in the result of transformFile |

Start in src/AjaxUploader.jsx at the headers passed to the upload request, and review the transformFile flow described in the issue. Update the README headers entry alongside the behavior. Done means headers can be supplied as a function receiving the transformed file, while object headers continue to work.