`uploadPath` fails without tailing slash

[marlun78]

var aws = JSON.parse(fs.readFileSync('./aws.json')),
    options = {
        uploadPath: '/site/js'
    };
gulp.src('./dist/file.js').pipe(s3(aws, options));

Results in the file being uploaded as: /site/jsfile.js instead of /site/js/file.js.