[form fields] fix severe memory leak due to @cached on field.input_name occuring on forms with persistent fields. Has been triggered in 3.6.1 on our public sites because: * the login form use persistent field since 3.6 * it's displayed on every anonymous request * another bug (fixed during the latest mini-sprint) made it also displayed for authenticated user see explanation in code for more detail. moral: be very careful with @cached...