Fixed #9893 -- Filename could exceed max_length on a collision

Passing max_length argument into the Storage's save() and get_available_name() methods.
Rewriting get_available_name() so filenames do not exceed max_length when there is a collision.
Adding test for the case.
I think `max_length` is a parameter here. Please see https://docs.python.org/3.3/faq/programming.html#faq-argument-vs-parameter for detailed information.