user_agent

User-Agent string to send with HTTP(S) requests, or null to use the default user agent, which is derived from the geocode-glib version and GApplication:id, for example: `geocode-glib/3.20 (MyAppId)`.

As per the [Nominatim usage policy](http://wiki.openstreetmap.org/wiki/Nominatim_usage_policy), it should be set to a string which identifies the application which is using geocode-glib, and must be a valid [user agent](https://tools.ietf.org/html/rfc7231#section-5.5.3) string.