import_headers fix with import_headers_result

lib/tallty_import_export/import.rb

@@ -97,7 +97,11 @@ module TalltyImportExport
     end
 
     def import_headers_result
-      klass.try(:headers) || klass.try(:import_headers) || klass.try(:model_headers) || (raise ArgumentError.new('missing import_headers_result'))
+      @headers || import_headers
+    end
+
+    def import_headers
+      @headers || klass.try(:import_headers) || klass.try(:model_headers) || (raise ArgumentError.new('missing import_headers'))
     end
 
     # 只保留 key, name, json, 合并到 import_header