Uses None instead of mutables for function param defaults

As seen on #1174809, changes use of mutable types as default
arguments and defaults them within the method. Otherwise, those
defaults can be unexpectedly persisted with the function between
invocations and erupt into mass hysteria on the streets.

There was indeed a test (TestSimpleClient.test_get_with_retries)
that was erroneously relying on this behavior. Since previous tests
had populated their own instantiations with a token, this test only
passed because the modified headers dict from previous tests was
being overridden. As expected, with the mutable defaults fix in
SimpleClient, this test begain to fail since it never specified any
token, yet it has always passed anyway. This change also now provides
the expected token.

Change-Id: If95f11d259008517dab511e88acfe9731e5a99b5
Related-Bug: #1174809