[BUGFIX] Remove Mutable Default Arguments

[johnjameswhitman]

Problem

We use this client and are debugging some stuff on our end. While tracing the code I noticed there are mutable default arguments for a few methods in the InterFAX client class, which can have some nasty surprises:
https://docs.python-guide.org/writing/gotchas/

Solution

Switch over to immutable default and populate with the corresponding list or dictionary as needed.

[johnjameswhitman]

Ran tests locally after making the change...

$ PYTHONPATH=. pytest tests
========================================================= test session starts ==========================================================
platform darwin -- Python 3.9.19, pytest-3.0.2, py-1.11.0, pluggy-0.3.1
rootdir: /Users/jwhitman/projects/third_party/github.com/johnjameswhitman/interfax-python, inifile: setup.cfg
collected 64 items

tests/test_account.py ..
tests/test_client.py ................
tests/test_documents.py ......
tests/test_files.py ......
tests/test_inbound.py .......
tests/test_outbound.py .........
tests/test_response.py ..................