Avoid instanceof for native object types

For classes defined in the module, this is fine.  For 'Error'
it's probably not very hazardous.  However, testing 'Object'
and 'String' is much more reliable using typeof, to work with
the repl and NODE_MODULE_CONTEXT modes.

lib/http.js

@@ -1437,12 +1437,12 @@ function getAgent(options) {
 
   var _opts = {};
 
-  if (options instanceof String) {
+  if (typeof options === 'string') {
     port = arguments[1] || 80;
     id = options + ':' + port;
     _opts.host = options;
     _opts.port = port;
-  } else if (options instanceof Object) {
+  } else if (options && options === 'object') {
     if (options.port || options.host) {
       host = options.host || 'localhost';
       port = options.port || 80;