Fixing an issue with rails 5 enums.

Don't use read_before_type_cast for enums as that will return a string for the enum key which is incorrect.

For example, this fixes failing tests when running:

   AR_VERSION=5.0 bundle exec rake test:mysql2

lib/activerecord-import/import.rb

@@ -339,10 +339,11 @@ def import_helper( *args )
           # this next line breaks sqlite.so with a segmentation fault
           # if model.new_record? || options[:on_duplicate_key_update]
             column_names.map do |name|
-              if model.class.column_defaults[name.to_s].is_a? Integer
-                model.read_attribute(name.to_s)
+              name = name.to_s
+              if model.class.column_defaults[name].is_a?(Integer) || defined_enums.has_key?(name)
+                model.read_attribute(name)
               else
-                model.read_attribute_before_type_cast(name.to_s)
+                model.read_attribute_before_type_cast(name)
               end
             end
           # end