stringfilter decorator is incompatible with is_safe / needs_autoescape flags

[Aymeric Augustin]

Defining a template filter like this doesn't work:

@register.filter
@stringfilter
def passthrough(value):
    return value
passthrough.is_safe = True

The problem is that the stringfilter decorator keeps a pointer to the underlying function, and the template engine uses it.

However, the last line only sets is_safe on the function returned by the decorator, not on the underlying function. As a consequence, the template filter doesn't know that the filter is safe.