fix: with_indifferent_access! do not have this method

lib/tallty_import_export/export.rb

@@ -165,7 +165,7 @@ module TalltyImportExport
         export_headers.select do |_header|
           _header.with_indifferent_access[:key].to_s.in?(headers_hash.keys)
         end.map do |_header|
-          _header.with_indifferent_access!
+          _header = _header.with_indifferent_access
           _header.merge(headers_hash[_header[:key]].delete_if { |k, v| v.blank? })
         end
       else

lib/tallty_import_export/import.rb

@@ -140,7 +140,7 @@ module TalltyImportExport
         import_headers.select do |_header|
           _header.with_indifferent_access[:key].to_s.in?(headers_hash.keys)
         end.map do |_header|
-          _header.with_indifferent_access!
+          _header = _header.with_indifferent_access
           _header.merge(headers_hash[_header[:key]].delete_if { |k, v| v.blank? })
         end
       else