Blob properties JSON serialization should include a real download url

When sending a DocumentModel over the wire the default JSON serialization will convert the Blob properties to a map.

For example :

{ 
  name: "somefile.txt", 
  data: "files/74498e6d-0562-4c66-bb35-04f8d75efc17?path=%2Fcontent",  
  length: 5, 
  encoding: null, 
  digest: "a2e4822a98337283e39f7b60acf85ec9", 
  mime-type: "text/plain"
}

There are at least 2 problems :

• the download link is not what we want
  • it binds to a WebEngine resource that we don't want to advertise as a download end point
• the JSON does not include a the repository name
  • processing the Blob JSON representation to wrap as a lazy downloadable stream is a pain since the property can not be handled separately from the Document