make the base_url agnostic to trailing slashes

[L0ric0]

at the moment if a base_url of the form https://example.com/ is given to the Client api calls to whoami will fail. This is because the path that is build looks like this https://example.com//_matrix/client/r0/account/whoami.

[tulir]

Yeah, it should use yarl's URL. I didn't know about that when I made this, which is why it's currently just concatenating strings :D