Broken on node v5.6.0

[mcollina]

bl tests are failing on node v5.6.0 because of nodejs/node#4951. What is failing is appending multiple buffer list within themselves: https://github.com/rvagg/bl/blob/master/test/test.js#L79-L94.

I am not sure we were relying on some broken functionality of core (not checking if arguments were buffers). In fact, the only thing of Buffer is relying on is a copy method.

Not sure what is the best fix for this, either on node or here.

Also reported here: moscajs/mosca#412.

cc @rvagg

[mcollina]

I think the best fix is to "recognize" if the passed in instance is a 'bl' instance, and append all the single buffers. This would be "introspecting" the passed instance, but I think that is better than allocating a big buffer for the sole purpose of appending it to another list.

The original code also might create issues when appending to the nested bl, so we should probably fix it here.