second attempt to add the VCF exporter

The VCF exporter finally generates output that can be used in R.
Is there any reason you are preparing the request yourself and not using the built in Django test client? Serrano has an example of making a request for a file [here](https://github.com/cbmi/serrano/blob/cb022657f6e9a364e73f4a45631e192becc3e46f/tests/cases/resources/tests/exporter.py#L56) and there is an example of sending parameters in a post request [here](https://github.com/cbmi/serrano/blob/830889abeb38efd131cd1c26312d73c0e49eabc8/tests/cases/resources/tests/context.py#L52-L69). By using the client, you shouldn't need to explicitly call write and you can test the full stack in that it will make sure all the URLs match and the request is routed to your exporter correctly and the correct file and file type is returned.