Improve reliability of BufferedIteratorProducer when handling strings

[ABCurado]

The implementation improves the existing code for the following reasons:

• The original code uses b''.join(buffer), which assumes that the buffer elements are bytes. If the iterator produces strings, this can lead to errors.
• The new code explicitly checks the type of buffer and converts it to bytes if necessary, ensuring correct handling of different data types.

[giampaolo]

BufferedIteratorProducer is used by listing commands (LIST, MLSD), which use AbstractedFS.format_list and AbstractedFS.format_mlsx. You can simply have these last 2 methods return bytes instead of strings (like they do). That's basically part of the contract between the fiilesystem class and the handler class.

I don't think it's worth it to modify BufferedIteratorProducer for this. In addition, isinstance() has some computational cost (not big but still...)