#1079: Fix parsing of encoded words.

This is a behavior change: before this leading and trailing spaces were
stripped from ASCII parts, now they are preserved.  Without this fix we didn't
parse the examples in the RFC correctly, so I think breaking backward
compatibility here is justified.

Patch by Ralf Schlatterbeck.
diff --git a/Lib/nntplib.py b/Lib/nntplib.py
index eb16206..2de6ebd 100644
--- a/Lib/nntplib.py
+++ b/Lib/nntplib.py
@@ -166,7 +166,7 @@
             parts.append(v.decode(enc or 'ascii'))
         else:
             parts.append(v)
-    return ' '.join(parts)
+    return ''.join(parts)
 
 def _parse_overview_fmt(lines):
     """Parse a list of string representing the response to LIST OVERVIEW.FMT