Add `options.rest.xml`, disable XML by default

Add an option `rest.xml` that quickly enables/disables support for responses in XML format.

The XML support is disabled by default now.

Versions `2.7.1` and newer has XML output enabled by default (see #111), which causes requests made from a browser to be handled with XML response type.

Such behaviour was considered as a breaking change when compared to `2.7.0`, see #118. This commit restores the behaviour of `2.7.0`.

/to @raymondfeng please review
/cc @ritch @gcirne @pungoyal FYI

**Note**

Once landed, we should release a new **minor** version and post a message to loopback mailing list to explain the situation, the consequences of this change and how to upgrade applications that are actually depending on the XML format.

**Upgrade instructions**

Modify you `server/config.json` and add `remoting.rest.xml: true`. Example:

```json
{
  "host": "0.0.0.0",
  "port": 3000,
  "remoting": {
    "rest": {
      "xml": true
    }
  }
}
```

@shelbys @dashby3000 GoDaddy will be affected by this change and have to modify their `server/config.json` file before installing a new strong-remoting version.