Request to 3rd party website in server logs

In my web server logs (I'm running node.js with express), intermixed with valid requests for files, I see the following:

[app-0 (out)] GET / 200 1ms - 3.95kb
[app-0 (out)] GET / 200 2ms - 3.95kb
[app-0 (out)] GET / 200 1ms - 3.95kb
---[app-0 (out)] GET http://www.mafengwo.com/ 200 1ms - 3.95kb
[app-0 (out)] GET / 304 1ms

I'm not the owner of "magengwo.com", and I'm unsure of how the attacker was able to query my web server for another website (and its a little more alarming that node responded with a 200). I would assume this is a bad thing, although I'm at a loss for how to reproduce this issue. Any help or explanation of whats going on here would be greatly appreciated