Use ContentDisposition instead of ContentType to control download

Fixes OpenRefine#1917. Previously we were using a funky ContentType to attempt
to force a file download rather than display in browser, but this
conflicted with attempts to save UTF-8 which was outside the Basic
Multilingual Plane (BMP).

By switching to ContentDisposition: attachment, which has been
the preferred method for a number of years, we can avoid this conflict.

main/src/com/google/refine/commands/project/ExportRowsCommand.java

@@ -100,6 +100,9 @@ public void doPost(HttpServletRequest request, HttpServletResponse response)
                 contentType = exporter.getContentType();
             }
             response.setHeader("Content-Type", contentType);
+            String path = request.getPathInfo();
+            String filename = path.substring(path.lastIndexOf('/') + 1);
+            response.setHeader("Content-Disposition", "attachment; filename=" + filename);
 
             if (exporter instanceof WriterExporter) {
                 String encoding = params.getProperty("encoding");

main/webapp/modules/core/scripts/project/exporters.js

@@ -112,10 +112,6 @@ ExporterManager.stripNonFileChars = function(name) {
 
 ExporterManager.handlers.exportRows = function(format, ext) {
   var form = ExporterManager.prepareExportRowsForm(format, true, ext);
-  $('<input />')
-  .attr("name", "contentType")
-  .attr("value", "application/x-unknown") // force download
-  .appendTo(form);
 
   document.body.appendChild(form);