souphttpsrc: add back "iradio-mode" property to disable sending of icecast request...

souphttpsrc: add back "iradio-mode" property to disable sending of icecast request headers

In 1.0 we now always send the icecast request headers by default, which
makes the server send icecasts metadata inserted into the stream if it
supports that. However, there are some use cases where this is not
desirable, like when just saving a radio stream to disk, so add back
the "iradio-mode" property to allow people to disable this.

https://bugzilla.gnome.org/show_bug.cgi?id=697984