Update mapbox result to use place_name as address

[tmh-dev]

Follow up to #1655

Non-nil values are now being returned (when appropriate) for the city, state, state_code, postal_code, country, country_code, and neighborhood methods. However, as a result the Geocoder::Result::Mapbox#address method needs to be updated otherwise the returned value will contain duplicated text.

Example: Given that the city method now returns a value for "place" type results, a search result for "Chicago, IL" will return "Chicago, Chicago, Illinois, United States". This is because .place_name and .city return the same value, and both are being used to build the address.

To solve this, we can simply use the place_name property on the Mapbox payload as the address rather than building our own.

Specs and fixtures have been added to test the different types of results you can get with Mapbox.

[alexreisner]

Excellent, thanks!