Can't set format to device_time as argument #311

Needed following 2 changes

1. Add format to device_time
   • https://github.com/appium/python-client/blob/master/appium/webdriver/webdriver.py
2. Enabled 'GET' to have body
   • https://github.com/SeleniumHQ/selenium/blob/a920faac5532175843c6a135f1f2dd621d8016c3/py/selenium/webdriver/remote/remote_connection.py#L393

For now, I'm not sure reason for No.2.
Need further checks.