text: Title: 登录失败报错，网页版可正常登录
  Body:  - 之前一直正常使用，定期重启
   - 扫码之后，等待时间很长
   - 之前出现过一次，手机上清理了缓存及一批好友之后正常
  ## Log
  ```
  Getting uuid of QR code.
  Downloading QR code.
  Please scan the QR code to log in.
  Please press confirm on your phone.
  Loading the contact, this may take a little while.
  Traceback (most recent call last):
    File "test.py", line 4, in <module>
      bot = Bot()
    File "/usr/local/lib/python3.5/site-packages/wxpy/api/bot.py", line 86, in __init__
      loginCallback=login_callback, exitCallback=logout_callback
    File "/usr/local/lib/python3.5/site-packages/itchat/components/register.py", line 35, in auto_login
      loginCallback=loginCallback, exitCallback=exitCallback)
    File "/usr/local/lib/python3.5/site-packages/itchat/components/login.py", line 67, in login
      self.get_contact(True)
    File "/usr/local/lib/python3.5/site-packages/itchat/components/contact.py", line 284, in get_contact
      seq, batchMemberList = _get_contact(seq)
    File "/usr/local/lib/python3.5/site-packages/itchat/components/contact.py", line 280, in _get_contact
      j = json.loads(r.content.decode('utf-8', 'replace'))
    File "/usr/local/Cellar/python3/3.5.1/Frameworks/Python.framework/Versions/3.5/lib/python3.5/json/__init__.py", line 319, in loads
      return _default_decoder.decode(s)
    File "/usr/local/Cellar/python3/3.5.1/Frameworks/Python.framework/Versions/3.5/lib/python3.5/json/decoder.py", line 339, in decode
      obj, end = self.raw_decode(s, idx=_w(s, 0).end())
    File "/usr/local/Cellar/python3/3.5.1/Frameworks/Python.framework/Versions/3.5/lib/python3.5/json/decoder.py", line 357, in raw_decode
      raise JSONDecodeError("Expecting value", s, err.value) from None
  json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
  ```
  