Including the resource once if an 'id' is present.

[renan]

Fix #84

From the Specification:

Each resource object SHOULD contain a unique identifier, or ID, when available.

[philsturgeon]

Awesome! Followup thought, could you add a test that tries to put the same bit of included data in twice (linked by two different parents) then prove it only gets placed in once?

[renan]

I've added this test: https://github.com/renan/fractal/compare/deep-includes

Which is now failing because the review author doesn't get included:

 Array (
     'book' => Array (...)
     'linked' => Array (
         'author' => Array (
             0 => Array (...)
-            1 => Array (...)
         )
         'reviews' => Array (...)
     )
 )

I tested with the code before the PR was applied and it was adding a linked key inside the already existing linked key:

             1 => Array (
-                'id' => 2
-                'name' => 'Bob'
+                'id' => 1
+                'name' => 'Dave'
             )
         )
         'reviews' => Array (...)
+        'linked' => Array (...)
     )
 )

I am not sure how to fix this.