Fix fields projections on nested sub-resources. Adds validation on fi…

[Dragomir-Ivanov]

…eld projection for Connected resources.

This PR helps with following scenario:

• Having this resource graph

	index := resource.NewIndex()
	aa := index.Bind("a", a, mem.NewHandler(), resource.Conf{AllowedModes: resource.ReadWrite})
	bb := aa.Bind("b", "id", b, mem.NewHandler(), resource.Conf{AllowedModes: resource.ReadWrite})
	bb.Bind("c", "id", c, mem.NewHandler(), resource.Conf{AllowedModes: resource.ReadWrite})

Being able to correctly execute queries like /a?fields=b{c}

[rs]

Had a quick glance and it LGTM. I'll let @smyrman do the final merge.

Sorry for the limited time I can dedicated to those PR atm.

[Dragomir-Ivanov]

Thanks guys. I think I have added everything I needed to rest-layer, so this may be my last patch for now. I will keep eye on the any issues on GitHub, and will support and try to help as much as I can.
Special thanks for @smyrman for his guidance and patience.

[Dragomir-Ivanov]

Guy, hold off with the merge. I thing there is some problem.

[Dragomir-Ivanov]

Okay, I think I fixed the issue with the latest patch.

[smyrman]

@Dragomir-Ivanov, when you are ready, please squash your commits to one and we can do a merge.

I would recommend the final commit message to follow the 50/72 rule, but won't review it in particular. This does however make the commit more readable in many tools, both CLI, web and GUI.

[Dragomir-Ivanov]

Done. Thanks for the link.