Only send opaque auth attribute if the server sent it with the challenge

Apache's mod_digest will not accept a client response that includes opaque unless the server also includes opaque. By default, mod_digest does not send opaque which means ActiveResource doesn't work with mod_digest out of the box.

This change modifies auth_attributes_for to only include opaque if it was not blank.
Move [].join

The [].join call was in the wrong spot which meant opaque was tagged onto the end incorrectly.

Fix tests.
threadsafe headers
Ensure belongs_to finder method sends a non-nil key
Add belongs_to tests for nil finder key case
Bump rails-observers required version
Adding `lib/activeresource.rb` to the project.

This allows to remove the `require: active_resource` from the Gemfile
and just use `gem 'activeresource'`.
Updating changelog.
Merge branch 'master' of github.com:allspark/activeresource