Update unit tests for stream_output to match the behaviour of a docker-py response.

Signed-off-by: Daniel Nephin <dnephin@gmail.com>

compose/progress_stream.py

@@ -1,8 +1,9 @@
-import codecs
 import json
 
 import six
 
+from compose import utils
+
 
 class StreamOutputError(Exception):
     pass
@@ -10,14 +11,13 @@ class StreamOutputError(Exception):
 
 def stream_output(output, stream):
     is_terminal = hasattr(stream, 'isatty') and stream.isatty()
-    if not six.PY3:
-        stream = codecs.getwriter('utf-8')(stream)
+    stream = utils.get_output_stream(stream)
     all_events = []
     lines = {}
     diff = 0
 
     for chunk in output:
-        if six.PY3 and not isinstance(chunk, str):
+        if six.PY3:
             chunk = chunk.decode('utf-8')
         event = json.loads(chunk)
         all_events.append(event)