Don't stream the writting in the export file

[kgonella]

We spotted an issue.

If you have a feed with a lot of products, the file can take time to generate.
If the shopping feed crawler try to read the file during the generation, the xml file is broken.

I suggest to write the feed in the temporary file (php://memory and php://temp ¶), then when the generation is finished, copy and replace the feed file.