Include `fastboot-body-start` script tag as part of `body` property in domContents() method in result.js

[dnalagatla]

In LinkedIn App, After bumping ember-cli-fastboot to v2.0.0 in , clear-double-boot failed as it was looking for script tag with id fastboot-body-start in Fastboot SSR content.

https://github.com/ember-fastboot/ember-cli-fastboot/blob/master/addon/instance-initializers/clear-double-boot.js#L10

We are using Fastboot library for SSR and access body by invoking method domContents() and stream that as response. As this body content doesn't include <script type="x/boundary" id="fastboot-body-start"></script>, clear double boot is failing in ember-cli-fastboot. I think this script tag should be part of body property.