My hosts file maps 127.0.0.1 to localhost.
$ curl -I \'localhost\'
curl: (7) Failed to connect to localhost port 80: Connection re
If anyone else comes across this and the accepted answer doesn't work (it didn't for me), check to see if you need to specify a port other than 80. In my case, I was running a rails server at localhost:3000 and was just using curl http://localhost, which was hitting port 80.
Changing the command to curl http://localhost:3000 is what worked in my case.