fix steam_guard passing and case login after passing login_cookies

[makarworld]

login() after passing login_cookies

Error case:

steam_client = SteamClient("API_KEY", login_cookies = cookies)
steam_client.login("USERNAME", "PASSWORD", "guard.json")    
if steam_client.is_session_alive():
    print('ok')

raise in .login() func

...steampy\client.py", line 105, in is_session_alive
    return steam_login.lower() in main_page_response.text.lower()
AttributeError: 'NoneType' object has no attribute 'lower'

because username not saved and steam_login still None

Note

also is_session_alive may check is self.username not None
or add func get_username() for get account login

---

passing steam_guard in __init__() function

Error case:

steam_client = SteamClient("API_KEY", "USERNAME", steam_guard = "guard.json", login_cookies = cookies)

raise

...steampy\client.py", line 30, in __init__
    self.steam_guard = guard.load_steam_guard(self.steam_guard)
AttributeError: 'SteamClient' object has no attribute 'steam_guard'

because guard.load_steam_guard() should take self.steam_guard_string as arg, not self.steam_guard.