nuxeo-uploader-behavior element should not rely on connection id

See PR.

Just a simple fix on nuxeo-uploader-behavior

if(!this.connection) {
   throw 'Missing connection';
 }

Property connection was not used and relying on an element id to use connection seems to limit the use of behavior.

For example, in nuxeo-web-ui, element nuxeo-dropzone.html the property is set in ready function :

ready: function() {
  this.connection = this.$.nx;
  ...
}