Allow .toJSON() function on objects to be used

[rhodgkins]

Firebase database itself allows toJSON to be used when storing data (I'm guessing through the use of JSON.stringify).

I've changed the .from method to check for objects and the presence of this method and use it.

Also updated any failing tests and added new ones to cover the change.

Note that this will mean that new Date() is now a valid value, stored as '2018-10-16T10:12:00.060Z' for example.

[rhodgkins]

Also added node 8 to test matrix - I've changed the node 6 check node --version | grep v6\. to not check for node 4 instead so 6 and 8 are now included.

I'm not sure why the builds are failing though - a basic npm t on node 6 and 8 work fine! (Node 4 build was failing before this change.)