Bugfix: WinVaultKeyring.get_credential with non-existent username returns credential of other user (#698)

[JamieBeverley]

Regarding #698

[JamieBeverley]

Question/Note:
Considered adding this additional assertion too (see previous commit) but it was failing intermittently on linux.

Sometimes keyring.get_credential(service, None).password would equate to password_2 and sometimes to password_1.

Sticking a time.sleep(1) between the 2 set_password calls above helped it reliably pass so perhaps there's some async/timing thing happening with dbus (can possibly look into that deeper but a bit out of my depths w/ dbus)

Suggested change

	assert keyring.get_credential(service, "user2").password == password_2
	assert keyring.get_credential(service, "user2").password == password_2
	# Most recent entered
	assert keyring.get_credential(service, None).password == password_2