Added option setContentDisposition

> Added option setContentDisposition, can be used to prevent setting a Content-Disposition header which might be disallowed on some server-side handlers.

So a few months ago, I ran into some issues trying to implement PUT uploads on Vimeo using jQuery File Upload. It looks like the `Content-Disposition` header, which is set on non-multipart uploads, is actually disallowed by Vimeo.
> [Error] XMLHttpRequest cannot load https://1511632926.cloud.vimeo.com/upload?[...]. Request header field Content-Disposition is not allowed by Access-Control-Allow-Headers. (90, line 0)

Here is a post on Stack Overflow where I explained how I "fixed" the issue : http://stackoverflow.com/questions/26222413/vimeo-api-streaming-upload-using-http-put-and-blueimps-jquery-fileupload
I thought it would be best to have a clean way to disable this feature, so I added an option `setContentDisposition`, it is set to `true` by default which preserves the original behaviour.