Add AWS 'key' form input element according to AWS documentation

http://doc.s3.amazonaws.com/proposals/post.html#Access_Control_Example

The above document explains that 'key' is a mandatory form element when
submitting POST requests to S3. I have added this element before 'file' since
the order matters:

https://forums.aws.amazon.com/message.jspa?messageID=131965
Merge branch 'master' of github.com:AFNetworking/AFAmazonS3Client