fix bug 990607: Ignore badly encoded referer headers when detecting search referer

Not sure if this is the best way to resolve this bug, but it at least stops [the error](https://rpm.newrelic.com/accounts/263620/applications/3172075/traced_errors/1535457945). The problem is that Python's `urlobject.URLObject` just does not gracefully handle a `Referer:` header that looks like this:
```
http://developer.mozilla.org/fr/docs/JavaScript/RÃ©fÃ©rence_JavaScript/RÃ©fÃ©rence_JavaScript/Objets_globaux/Date/parse
```