use unicode_to_str with readline.add_history

mutliline PR  #838 added a call to readline.add_history, but without
the unicode/str protection used elsewhere, allowing `In[1]: u'é'` to crash IPython.

This makes the call match others in core.interativeshell.

IPython/frontend/terminal/interactiveshell.py

@@ -235,7 +235,9 b' class TerminalInteractiveShell(InteractiveShell):'
             hlen = self.readline.get_current_history_length()
             for i in range(hlen - hlen_before_cell):
                 self.readline.remove_history_item(hlen - i - 1)
-            self.readline.add_history(source_raw.rstrip())
+            stdin_encoding = sys.stdin.encoding or "utf-8"
+            self.readline.add_history(py3compat.unicode_to_str(source_raw.rstrip(),
+                                        stdin_encoding))
 
     def interact(self, display_banner=None):
         """Closely emulate the interactive Python console."""