add missing block key in complete_request

IPython/html/static/services/kernels/js/kernel.js

@@ -339,6 +339,7 b' var IPython = (function (IPython) {'
         var content = {
             text : '',
             line : line,
+            block : null,
             cursor_pos : cursor_pos
         };
         var msg = this._get_msg("complete_request", content);