fix: export_association_headers array convert

lib/tallty_import_export/export.rb

@@ -69,7 +69,9 @@ module TalltyImportExport
         if respond_to?(:export_association_headers)
           records.each do |record|
             context.record = record
-            Array(export_association_headers(**options)).each do |association_headers|
+            _export_association_headers = export_association_headers(**options)
+            _export_association_headers = _export_association_headers.is_a?(Array) ? _export_association_headers : [_export_association_headers]
+            _export_association_headers.each do |association_headers|
               association_headers.with_indifferent_access!
               association_records = record.send(association_headers[:association])
               sheet_name = if association_headers[:sheet_name].present?