Allow method_decorator to handle multiple functions

[Alexey Kotenko]

Allowing method_decorator to handle multiple function names would avoid invoking it multiple times:

@method_decorator(login_required, name='get')
@method_decorator(login_required, name='post')
class SimpleView(View):
    ...

Instead of this lets modify function to polymorpic style:

@method_decorator(login_required, name=['get', 'post'])
class SimpledView(View):
    ...