Skypeweb transport does not deliver private muc messages.

[Ri0n]

backend logs for one message

7159: 2017-06-19 17:51:49,352 INFO  libpurple: http:Performing new request 0x1f97400 to co4-client-s.gateway.messenger.live.com.
7159: 2017-06-19 17:51:49,393 INFO  backend: Sending message to '19:de2d01a5eba64cc06256680f0499c83f@thread.skype/kva.vg'
7159: 2017-06-19 17:51:49,393 INFO  libpurple: http:Performing new request 0x1ff57f0 to co4-client-s.gateway.messenger.live.com.
7159: 2017-06-19 17:51:49,587 INFO  libpurple: http:Request 0x1f97400 performed without success.
7159: 2017-06-19 17:51:49,628 INFO  libpurple: http:Request 0x1ff57f0 performed without success.
7159: 2017-06-19 17:51:49,629 INFO  libpurple: skypeweb:executing callback for https://co4-client-s.gateway.messenger.live.com/v1/users/ME/conversations/1%3A19%3Ade2d01a5eba64cc06256680f0499c83f%40thread.skype%2Fkva.vg/messages