gdrive: don't stream response for gdoc downloads
gdrive: don't stream response for gdoc downloads

 * Google Drive now reports that .gdoc files have a size of 1kb.  This
   was confusing the `download` method, b/c .gdoc files cannot be
   downloaded directly; they must be exported into a supported format.
   WB doesn't know the size of the exported file ahead of time and
   needs to download the full file so it can calculate it.  Stop using
   the heuristic of "undefined file size == gdoc file" and instead use
   the `.is_google_doc` property of the file metadata object.

felliott/waterbutler,CenterForOpenScience/waterbutler