Fix a bug that google mobile crawler causes ValueError

```
'SoftBank/1.0/920SH/SHJ001 Browser/NetFront/3.4 Profile/MIDP-2.0 Configuration/CLDC-1.1 AppEngine-Google; (+http://code.google.com/appengine; appid: s~google.com:mobile-checker)'
```

というUser-AgentがValueErrorになるバグを修正。