Set "share_with" field to the ID of the circle

When a share is shared with a circle the "share_with" field returned by
the API endpoint was always set to the name of the circle. However, the
name is not enough to identify a circle. The Circles app now provides
the ID of the circle in the "shared with" field of a Share, so this
commit modifies the API endpoint to set the "share_with" field to the ID
of the circle when provided by the Circles app.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>