Bugfix Us geocoder

``` ruby
GeoKit::Geocoders::UsGeocoder.geocode("sf")
```

Produces:

```
TypeError: can't convert TypeError into String
from /home/bogdan/.rvm/gems/ruby-1.9.3-p194/bundler/gems/geokit-e712f0a7b21a/lib/geokit/services/us_geocoder.rb:44:in `+'
```

I believe this line never worked well.