Why can't I receive plain text sent in a POST request body?

The request made from a client browser:

var xhr = new XMLHttpRequest();
xhr.open("POST", "/MyRoute/MySubRoute");
xhr.setRequestHeader("Content-Type", "text/plain;charset=UTF-8");
xhr.send("hello!");

Using Express with my node server:

app.post('/MyRoute/MySubRoute', function(req, res) {
    console.log("Received:"+require('util').inspect(req.body,{depth:null});
    res.send();
});

Logged to the console I get:

Received:{}

I've tried with text/plain (no charset), with the same result. If I change my content type to application/json and pass a simple JSON string it works fine.

node.js http post
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