Test mutable arguments in function definition.

Hi, I wanted to add a test against the problem of the mutable default argument which can cause unexpected behaviour.

* http://docs.python-guide.org/en/latest/writing/gotchas/#mutable-default-arguments
* http://pythonconquerstheuniverse.wordpress.com/2012/02/15/mutable-default-arguments/

Is pyflakes the right place to do so? How can I make it better? Thanks