Read MaxMind service name from config

instead of inferring from response length. This is less than ideal but
it fixes issue #396 and, given what appears to be MaxMind's inconsistent
response lengths, it may not be possible to do much better.

lib/geocoder/results/maxmind.rb

@@ -67,13 +67,12 @@ module Geocoder::Result
 
     ##
     # Name of the MaxMind service being used.
-    # Inferred from format of @data.
     #
     def service_name
-      self.class.field_names.to_a.each do |n,f|
-        return n if (@data.size..@data.size+1).include?(f.size)
-      end
-      nil
+      # it would be much better to infer this from the length of the @data
+      # array, but MaxMind seems to send inconsistent and wide-ranging response
+      # lengths (see https://github.com/alexreisner/geocoder/issues/396)
+      Geocoder.config.maxmind[:service]
     end
 
     def field_names

test/services_test.rb

@@ -171,27 +171,27 @@ class ServicesTest < Test::Unit::TestCase
   end
 
   def test_maxmind_result_knows_country_service_name
-    Geocoder.configure(:ip_lookup => :maxmind)
+    Geocoder.configure(:ip_lookup => :maxmind, :maxmind => {:service => :country})
     assert_equal :country, Geocoder.search("24.24.24.21").first.service_name
   end
 
   def test_maxmind_result_knows_city_service_name
-    Geocoder.configure(:ip_lookup => :maxmind)
+    Geocoder.configure(:ip_lookup => :maxmind, :maxmind => {:service => :city})
     assert_equal :city, Geocoder.search("24.24.24.22").first.service_name
   end
 
   def test_maxmind_result_knows_city_isp_org_service_name
-    Geocoder.configure(:ip_lookup => :maxmind)
+    Geocoder.configure(:ip_lookup => :maxmind, :maxmind => {:service => :city_isp_org})
     assert_equal :city_isp_org, Geocoder.search("24.24.24.23").first.service_name
   end
 
   def test_maxmind_result_knows_omni_service_name
-    Geocoder.configure(:ip_lookup => :maxmind)
+    Geocoder.configure(:ip_lookup => :maxmind, :maxmind => {:service => :omni})
     assert_equal :omni, Geocoder.search("24.24.24.24").first.service_name
   end
 
   def test_maxmind_special_result_components
-    Geocoder.configure(:ip_lookup => :maxmind)
+    Geocoder.configure(:ip_lookup => :maxmind, :maxmind => {:service => :omni})
     result = Geocoder.search("24.24.24.24").first
     assert_equal "Road Runner", result.isp_name
     assert_equal "Cable/DSL", result.netspeed