fix; fix arguments

lib/tallty_import_export/import.rb

@@ -136,7 +136,7 @@ module TalltyImportExport
     end
 
     def process_options(options)
-      options = import_options.merge(options).with_indifferent_access
+      options = import_options.merge(options).with_indifferent_access.symbolize_keys
       @headers = options.delete(:headers) || import_headers(**options)
       @primary_keys = options.delete(:primary_keys) || @headers.map { |header| header[:primary_key] ? header[:key].to_sym : nil }.compact
       @skip_keys = options.delete(:skip_keys) || @headers.map { |header| header[:skip] ? header[:key].to_sym : nil }.compact