Fix nioBuffer implementation for CompositeByteBuf

The method for copying all of a CompositeByteBuf's constituent buffers into a single nio buffer had a bug. I've fixed the bug and included a test that previously failed. 

I was unsure (and left a note to that effect) of what the behavior of CompositeByteBuf#addComponent should be. It does not appear to bump the writer index, resulting in 0 readable bytes after components have been added. Is it the intention that the code adding the components be responsible for updating the writer index?