Add mediaType to ActivityStreams representation
ActivityStreams says the content property is assumed to be text/html format. https://www.w3.org/TR/activitystreams-vocabulary/#dfn-content
Gancio seems to use plain text which is fine, but doesn't add a mediaType property with value text/plain to explicit that.
As for summary , it seems only HTML is allowed for some reason (which sounds odd to me). https://www.w3.org/TR/activitystreams-vocabulary/#dfn-summary