Server URL with trailing / produces unexpected error

[pinusc]

Currently, if the url parameter in the config has a trailing slash (e.g. https://music.example.com/ as opposed to https://music.example.com), then subidy is able to connect correctly and even show songs/albums/etc, but fails to stream - yielding instead a very confusing error about needing a gstreamer plugin for text/html.

The reason for this is that subidy later generates URLs with double slashes, such as https://music.example.com//rest/stream.view?id=tr-12749&c=Mopidy-Subidy&v=1.14.0&u=******&p=******, which can produce an HTML error page instead of a binary audio stream.

I made a PR to fix this, it should link to this issue.