Preserve "data" and "__schema" keys when outputting JSON schema from schema:download

[jamesmbourne]

This resolves #555. The JSON schema generated by schema:download did not have a root node of:

"data": {
	"__schema" :{
		"queryType": ...
		...
	}
}

Snapshot tests for the output schema have been updated where appropriate.

[apollo-cla]

@jamesmbourne: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Meteor Contributor Agreement here: https://contribute.meteor.com/

[martijnwalraven]

Thanks a lot for fixing this, looks good to me!