feat: use dig instead of fetch

lib/tallty_import_export/import.rb

@@ -116,13 +116,13 @@ module TalltyImportExport
       import_header_hash = import_headers.to_h { |header| [header.with_indifferent_access[:key], header] }.with_indifferent_access
       @headers.map do |header|
         key = header[:key]
-        if import_header_hash.fetch(key)&.fetch(:proc).present?
-          header[:proc] = import_header_hash.fetch(key)&.fetch(:proc)
+        if import_header_hash.dig(key, :proc).present?
+          header[:proc] = import_header_hash.dig(key, :proc)
         end
         header
       end
-    # rescue
-    #   @headers
+    rescue
+      @headers
     end
 
     def import_options