Optimize sample XML response for most browser
Optimize sample XML response for most browser

PhantomJS 2.1.1 on Ubuntu responds with

```
<?xml version="1.0" encoding="UTF-8"?>
<message>
  <from>Alice</from>
  <to>Bob</to>
  <body>Hello</body>
</message>
```

When he fetches this XML :

```
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<message>
  <from>Alice</from>
  <to>Bob</to>
  <body>Hello</body>
</message>
```

By adding the standalone attribute to the xml tag, we make the response
identical on macOS (dev / test) and Linux (CI / production).

levups/http-tester