Detect XMLHttpRequest before detecting node.

There have been two recent incidents wherein the node-sniffing code
caused an issue.  This alternatve allows an existing implementation
of XMLHttpRequest to be used.

I recently hit this issue inside a cordova-based mobile webview emulator.  This change fixed it.  I don't know if this fixes the issue for everyone, but I thought I'd throw this out there to see what people think.  Especially @scothis. :)