Allow arbitrary attributes on Geocoder::Result::Test

Similar to #565, but allows to define any attribute methods on test result without changing the code. For example:

``` ruby
Geocoder.configure(:lookup => :test)
Geocoder::Lookup::Test.set_default_stub([{    
      'city'      => 'New York',
      'police'   => 'NYPD'
}])

Geocoder.search('New York').first.police # => 'NYPD'
```