Correctly pass headers to embedded objects

[fancyremarker]

The added test explains the problem case, but basically any headers set on the initial HyperResource object were not being passed to embedded objects. (Link objects got the headers just fine.)

I hit this when passing an OAuth 2.0 bearer token. It worked fine for link resources, but I was getting 401 Unauthorized for any embedded resources.

The situation affects other client headers, like Content-Type.

options = { headers: { 'Authorization' => "Bearer #{access_token}" } }
api = HyperResource.new(options)
api.get.things.get
# => (A-OK)
api.get.things.first.get
# !! HyperResource::ClientError: 401 (unauthorized)

[gamache]

This is wonderful. Great catch, and thanks for the patch!