Added fixes suggested by @rodrigods

During his code review of commit 14b19855f7fc9732064c7f2b6714028f3e790e95
Merge branch 'master' of github.com:buddycloud/buddycloud-http-api
POST /:channel/subscribers/:node json key is simply the jid

My code was assuming that the json passed within the request would have
keys composed by jid/node, but only the jid is necessary.