[FIX] web_graph: force row title to be string

If the title was 'true' or 'false', the export failed, because we are tying to
concat bool and str. TypeError: cannot concatenate 'str' and 'bool' objects

How to reproduce, install sale, be sure to have one order with delivered = True
In Reporting / Sale analysis, add a group by 'shipped' filed to have 'true' as
title.

This commit closes issue odoo/odoo#13425

addons/web_graph/controllers/main.py

 from openerp import http
 import simplejson
+from openerp.tools import ustr
 from openerp.http import request, serialize_exception as _serialize_exception
 from cStringIO import StringIO
 from collections import deque
@@ -70,7 +71,7 @@ class TableExporter(http.Controller):
         # Step 3: writing data
         x = 0
         for row in jdata['rows']:
-            worksheet.write(y, x, row['indent'] * '     ' + row['title'], header_plain)
+            worksheet.write(y, x, row['indent'] * '     ' + ustr(row['title']), header_plain)
             for cell in row['cells']:
                 x = x + 1
                 if cell.get('is_bold', False):