Delegate primary_key to superclass when necessary, otherwise default

## Current Behavior
The primary_key attribute does not delegate to the superclass when read; causing the following to happen:
```ruby
class BaseResource < ActiveResource::Base
  self.primary_key = :custom_key
end

class DerivedResource < BaseResource
end

BaseResource.primary_key    => :custom_key
DerivedResource.primary_key => 'id'
```
## New Behavior
Simply delegate the primary_key attribute to the superclass if it isn't set. Thus becoming:
```ruby
BaseResource.primary_key    => :custom_key
DerivedResource.primary_key => :custom_key
```