documents: Retain the content type when adding an entry to a folder
documents: Retain the content type when adding an entry to a folder

Otherwise, it will be identified as application/octet-stream and
exportLinks will be missing.

https://bugzilla.gnome.org/show_bug.cgi?id=684920

pwithnall/libgdata,pwithnall/libgdata,GNOME/libgdata