Forms: Assert that js and css attributes of Media contain list or tuple

[Thomas Güttler]

Hi,

this patch checks if the attributes js and css of the Media class contain a list or tuple, and not
a string.

Test included.

[Jacob]

isinstance() checks are not good Python -- any iterable should work for js, for example -- and this check isn't needed. If someone uses a bad type, they'll get an error. That's how Python works.

[Thomas Güttler]

Strings are iterable. But if you use a string, you won't get the expected result.

A patch like this would make django more friendly for people with little experience.

Maybe I should have done it different: Check if the object is iterable, but is not a string.