Return OSM main tag information in geocodejson. This is not part
of the official spec but can be useful to get more detailed information
of the object type. Brings the Nominatim output closer to what
Photon produces.
'type' so far contained the value of the OSM tag. That is rarely
helpful because it is not a restricted class of values. Change
this to contain the types as defined in the geocodejson spec,
which correspond to the address layer names.
When name and address is empty, the keywords field in the response
of the details API would be an array because that is what PHP's
json_encode defaults to with empty array(). This default can only
be changed globally per json_encode call and that might cause
unintended colleteral damage. Work around the issue by making
name and address an empty array instead of keywords.
Fixes#2329.
