Fix for #6464

When checking the defaults dict the keys "m_name" and "m_fun" are already renamed to "name" and "fun" earlier in the code so use arg (="name") instead of rarg (="m_name")

This solves #6464

This is my first contribution so I certainly need some adult supervision...

On a side note:
As of python 2.6 inspect.getargspec returns a named tuple
http://docs.python.org/2.6/library/inspect.html#inspect.getargspec

Would it be ok if I in another branch and pull request changed:
aspec[0] -> aspec.args
aspec[1] -> aspec.varargs
aspec[2] -> aspec.keywords
aspec[3] -> aspec.defaults
To get a more readable code?
I might be getting old, but it hurt my brains trying to remember which index refers to which argument when trying to figure out what the code does.