I don't see a good way to implement a custom dat file that works with the vendor's libraries. It's fairly proprietary. 

That line isn't really necessary though.
The rest of it is only there to satisfy the default value from 
geoip_decoder.go line 41: DatabaseFile:   filepath.Join(Globals().ShareDir,"GeoLiteCity.dat"),

But the database file is not actually getting opened.
It's not a great test but might be better than nothing if we can't stub out our own database?