drive: fix server side copy of big files

Before this change rclone was sending a MimeType in the requests for
server side Move and Copy.

The conjecture is that if you attempt to set the MimeType to something
different in a Copy then Google Drive has to do an actual copy of the
file data.  This takes a very long time (since it is large) and fails
after a 90s timeout.

After the change we no longer set the MimeType in Move or Copy and the
copies happen instantly and correctly.

Many thanks to @darthShadow for discovering that this was causing the
problem.

Fixes #3070
Fixes #3033
Fixes #3300
Fixes #3155

backend/drive/drive.go

@@ -1649,7 +1649,6 @@ func (f *Fs) createFileInfo(ctx context.Context, remote string, modTime time.Tim
 		Name:         leaf,
 		Description:  leaf,
 		Parents:      []string{directoryID},
-		MimeType:     fs.MimeTypeFromName(remote),
 		ModifiedTime: modTime.Format(timeFormatOut),
 	}
 	return createInfo, nil
@@ -1713,6 +1712,8 @@ func (f *Fs) PutUnchecked(ctx context.Context, in io.Reader, src fs.ObjectInfo,
 	}
 	if importMimeType != "" {
 		createInfo.MimeType = importMimeType
+	} else {
+		createInfo.MimeType = fs.MimeTypeFromName(remote)
 	}
 
 	var info *drive.File