Provided response_stream ends up in query

[laborg]

When trying to make use of the response_stream it gets fed into the query:

io = Base.BufferStream()
S3.get_object(bucket_name, file_name, Dict("response_stream"=>io, "return_stream"=>true))

...leads to:

ERROR: MethodError: no method matching iterate(::Base.BufferStream)

[laborg]

fixed by proposed #321