documents: Add support for updating content using Drive v2
documents: Add support for updating content using Drive v2

There is a bug in the Drive web UI where the preview doesn't get
updated with the new content, but if you download or go through the
revisions then things are as they should be. I could reliably reproduce
it with plain/text content.

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

GNOME/libgdata,pwithnall/libgdata,pwithnall/libgdata