Add ability to send bytes to lewis

[John-Holt-Tessella]

When the stream interface is sent a byte stream in tried to encode it into a string which does not work. Instead we only encode when the input type is a string.

[DominicOram]

@mattclarke are you happy for me to merge this? It maintains backwards compatibility

[mattclarke]

I was planning to take a look this afternoon, but now I have seen how short it is I'll do it now... ... ... Done!

[DominicOram]

Thanks @mattclarke :) Hope all is well with you!