add prefer_stream arg to utils.encoding.getdefaultencoding

sometimes the stream encoding should be ignored,
and start with locale (e.g. Popen on Windows Python 2).

Default behavior is unchanged.

IPython/utils/encoding.py

@@ -35,16 +35,20 b' def get_stream_enc(stream, default=None):'
 # to match the environment.
 # Defined here as central function, so if we find better choices, we
 # won't need to make changes all over IPython.
-def getdefaultencoding():
+def getdefaultencoding(prefer_stream=True):
     """Return IPython's guess for the default encoding for bytes as text.
-
-    Asks for stdin.encoding first, to match the calling Terminal, but that
-    is often None for subprocesses.  Fall back on locale.getpreferredencoding()
+    
+    If prefer_stream is True (default), asks for stdin.encoding first,
+    to match the calling Terminal, but that is often None for subprocesses.
+    
+    Then fall back on locale.getpreferredencoding(),
     which should be a sensible platform default (that respects LANG environment),
     and finally to sys.getdefaultencoding() which is the most conservative option,
-    and usually ASCII.
+    and usually ASCII on Python 2 or UTF8 on Python 3.
     """
-    enc = get_stream_enc(sys.stdin)
+    enc = None
+    if prefer_stream:
+        enc = get_stream_enc(sys.stdin)
     if not enc or enc=='ascii':
         try:
             # There are reports of getpreferredencoding raising errors