connection: always set pmuc-v1 capability when sending presence
connection: always set pmuc-v1 capability when sending presence

If you are invited to a Google private MUC, then google's server
(talk.google.com) will get your caps and if you have the "pmuc-v1" cap
set as an entity capability element, it will invite you like any
normal MUC.

Partly ixes fd.o#22768 (as long as discoing chatrooms is
disabled). This commit enables us to be invited to private MUCs by the
Google client, but doesn't currently support creating them without
making the UI terrible.

Signed-off-by: Jonny Lamb <3d1fb3642aaab2ff70e3392a5ab916bf99eed6a3@collabora.co.uk>

community-ssu/telepathy-gabble,community-ssu/telepathy-gabble,community-ssu/telepathy-gabble,community-ssu/telepathy-gabble