templater: apply "stringify()" on sub expression to get string correctly Templating syntax allows nested expression to be specified as parts below, but they are evaluated as a generator and don't work correctly. - 'sep' of 'join(list, sep)' - 'text' and 'chars' of 'strip(text, chars)' In the former case, 'sep' returns expected string only for the first separation, and empty one for the second or later, because the generator has only one element. In the latter case, templating is aborted by exception, because the generator doesn't have 'strip()' method (as 'text') and can't be passed as the argument to 'str.strip()' (as 'chars'). This patch applies "stringify()" on these sub expression to get string correctly.