Ensure the #max_allowed_packet method exists for the MysqlAdapters.

https://github.com/zdennis/activerecord-import/issues#issue/12

lib/activerecord-import/adapters/mysql_adapter.rb

@@ -6,7 +6,16 @@ module ActiveRecord::Import::MysqlAdapter
         include ActiveRecord::Import::OnDuplicateKeyUpdateSupport
       end
     end
-  
+    
+    # Returns the maximum number of bytes that the server will allow
+    # in a single packet
+    def max_allowed_packet # :nodoc:
+      result = execute( "SHOW VARIABLES like 'max_allowed_packet';" )
+      # original Mysql gem responds to #fetch_row while Mysql2 responds to #first
+      val = result.respond_to?(:fetch_row) ? result.fetch_row[1] : result.first[1]
+      val.to_i
+    end
+    
     # Returns a generated ON DUPLICATE KEY UPDATE statement given the passed
     # in +args+. 
     def sql_for_on_duplicate_key_update( table_name, *args ) # :nodoc: