[3.6]bpo-15786: Fix IDLE autocomplete return problem. (#2198) (#2199)

Before,  Enter would not, for instance, complete 're.c' to 're.compile' even with 'compile' highlighted.  Now it does.  Before, '\n' was inserted into text, which in Shell meant compile() and possibly execute.  Now cursor is left after completion.
(cherry picked from commit 32fd874afe)

Lib/idlelib/autocomplete_w.py

@@ -325,8 +325,9 @@ class AutoCompleteWindow:
             return "break"
 
         elif keysym == "Return":
+            self.complete()
             self.hide_window()
-            return None
+            return 'break'
 
         elif (self.mode == COMPLETE_ATTRIBUTES and keysym in
               ("period", "space", "parenleft", "parenright", "bracketleft",