400 should be 404

lib/document_handler.js

@@ -18,7 +18,7 @@ DocumentHandler.prototype.handleGet = function(key, response) {
   }
   else {
     winston.warn('document not found', { key: key });
-    response.writeHead(400, { 'content-type': 'application/json' });
+    response.writeHead(404, { 'content-type': 'application/json' });
     response.end(JSON.stringify({ message: 'document not found' }));
   }
 };