S3 PutCopy: urlencode source string

As specified in http://docs.aws.amazon.com/AmazonS3/latest/API/RESTObjectCOPY.html, the x-amz-copy-source-string must be url-encoded. Otherwise, URLs containing accents etc will produce a 403 with the following message:

`The request signature we calculated does not match the signature you provided. Check your key and signing method.`