* Fix root path requests
Since #5759 we've tried to access the path explicitly. However, this
raises KeyError exception when trying to access the proxied root path
(e.g. http://supervisor/core/api/). Before #5759 get was used, which
lead to no exception, but instead inserted a `None` into the path.
It seems aiohttp doesn't provide a path when the root is accessed. So
simply convert this to no path as well by setting path to an empty
string.
* Add rudimentary pytest for regular proxy requets