setting appropriate content-type

[Yonik Seeley]

So, right now, the Solr servlet always sets a content type of
"text/xml;charset=UTF-8" before getting the Writer to write the
response.

Now that we have other response writers (like JSON) it seems like they
need a say over what the content-type is.... so the new servlet code
would look something like this:

       QueryResponseWriter responseWriter =
core.getQueryResponseWriter(solrReq);
       response.setContentType(
responseWriter.getContentType(solrReq, solrRsp) );
       PrintWriter out = response.getWriter();
       responseWriter.write(out, solrReq, solrRsp);

So the new method is QueryResponseWriter.getContentType():String, and
I also pass the request and response so content-type could change
based on either of those.

Make sense?  Any other suggestions?

-Yonik