Encode using 'replace' for set_next_input.

IPython/core/interactiveshell.py

@@ -1704,7 +1704,7 b' class InteractiveShell(SingletonConfigurable, Magic):'
         [D:\ipython]|2> Hello Word_  # cursor is here        
         """
         if isinstance(s, unicode):
-            s = s.encode(self.stdin_encoding)
+            s = s.encode(self.stdin_encoding, 'replace')
         self.rl_next_input = s
 
     # Maybe move this to the terminal subclass?