import_headers_result with @headers setting

lib/tallty_import_export/import.rb

@@ -79,7 +79,7 @@ module TalltyImportExport
 
     def convert_data line_info
       line_info.with_indifferent_access.reduce({}) do |h, (k, v)|
-        header = @headers.find do |_header|
+        header = import_headers_result.find do |_header|
           _header[:key].to_sym == k.to_sym
         end
         # header[:convert] = handle_xxx
@@ -104,7 +104,7 @@ module TalltyImportExport
     end
 
     def import_headers_result
-      @headers || import_headers
+      @headers ||= import_headers
     end
 
     def import_headers **args