Add MD5-Header option to CLDUploadRequestParams #390

Using OPTIONS, you can see that Cloudinary accepts Content-MD5 as a header when uploading a file. Currently, there is no way to add this or any other headers to a CLDUploadRequest. The Content-MD5 header seems like an important header to allow when uploading from iOS. The Cloudinary Rails/Active storage integration requires the MD5 be present, so this would bring the iOS client up to parity with the Ruby gem.