Set room notification JSON POST body to UTF-8

Without this change, room notification POSTs being sent to HipChat's REST API containing non-ASCII characters will fail to work, as a decoding issue prevents the notifications from being accepted.  The JSON specification indicates that the application/json media-type should always be encoded in Unicode, but the single-arg StringEntity() constructor assumes the text/plain media type, which defaults to encoding as ISO-8859-1, as indicated in the HTTP specification.