Prevent metadata update of blobs managed with liveconnect

The default binary metadata mapping breaks the Liveconnect link for pictures if dc:description or dc:source is updated on the document.

Step to reproduce:

• Create a new Picture document and add a picture file (jpeg) from google drive or another Liveconnect provider
• Edit the document and set dc:description or dc:source
• Save
• Refresh
• The document now contains a local blob instead of the google drive one