Some tests in test_account_quotas.py registered a response with
FakeSwift during setUp and then subsequently mutated the response's
headers dict. Worse, some tests then mutate the headers again and
re-use the same response.
This relies on the FakeSwift implementation not copying the original
headers when the response is registered, which may not remain the case
(as the author discovered with the Related-Change). It's also an
unnecessary shortcut.
Change-Id: I3217c17936d0c11b03de4a4a172bb2fb0a2be734
Related-Change: I84604a7ea049850ad4d0f0cea2096ab8a98dfb4a
6a5a681227