media: Fix attribute escaping for GDataMediaGroup
media: Fix attribute escaping for GDataMediaGroup

Note: we can't add a test case for this, as GDataMediaGroup isn't publicly
exposed.

Helps: bgo#631033

Pecisk/libgdata,pwithnall/libgdata,Pecisk/libgdata-gtasks,GNOME/libgdata,Pecisk/libgdata,Distrotech/libgdata,Pecisk/libgdata,Distrotech/libgdata,Distrotech/libgdata,pwithnall/libgdata,Pecisk/libgdata-gtasks,Pecisk/libgdata-gtasks