Add test for missing Accept header.

Author: RubenVerborgh

test/unit/resource-mapper-test.js

@@ -99,6 +99,15 @@ describe('ResourceMapper', () => {
       ],
       new Error('Not acceptable'))
 
+    itMapsUrl.skip(mapper, 'an extensionless URL of an existing file, no choice given',
+      {
+        url: 'http://localhost/space/foo'
+      },
+      [
+        `${rootPath}space/foo.txt`
+      ],
+      { path: `${rootPath}space/foo.txt` })
+
     // Security cases
 
     itMapsUrl(mapper, 'a URL with an unknown content type',