Fix 304 causes error

304 cause error where would try to check headers when they were
undefined.

lib/server.js

@@ -129,10 +129,13 @@ module.exports = function(opts){
 					return;
 				}
 
-                res.header(headers);
-                if(headers['Content-Type'] === 'application/json') {
-                    return res.jsonp(tile);
-                }
+				if(headers) {
+                    res.header(headers);
+                    if(headers['Content-Type'] === 'application/json') {
+                        return res.jsonp(tile);
+                    }
+				}
+
                 return res.send(tile);
 			}
 		);