fix raw_input in qtconsole

adds a flush, whose absence produced mis-measured prompts,
including the prompt in the raw_input reply.

closes  #3785

IPython/qt/console/console_widget.py

@@ -2060,6 +2060,7 b' class ConsoleWidget(LoggingConfigurable, QtGui.QWidget):'
                 self._prompt = prompt
                 self._prompt_html = None
 
+        self._flush_pending_stream()
         self._prompt_pos = self._get_end_cursor().position()
         self._prompt_started()