[rackspace|identity] Correctly populate model from request data.

Previously, the model was being incorrectly loaded from request data. As a
result, the username and apiKey properties were always nil. This fixes parsing
and improves the tests to catch this type of bug in the future.

lib/fog/rackspace/models/identity/credentials.rb

@@ -17,14 +17,15 @@ def all
 
         def get(id)
           requires :user
-          data = retrieve_credentials.find{ |credential| credential['id'] == id }
+          data = retrieve_credentials.find { |credential| credential['apiKey'] == id }
           data && new(data)
         end
 
         private
 
         def retrieve_credentials
-          data = service.list_credentials(user.identity).body['credentials']
+          raw_credentials = service.list_credentials(user.identity).body['credentials']
+          raw_credentials.map { |c| c['RAX-KSKEY:apiKeyCredentials'] }
         end
       end
     end

tests/rackspace/models/identity/credentials_tests.rb

@@ -5,12 +5,21 @@
   service = Fog::Rackspace::Identity.new
   user = service.users.all.first
 
-  tests("#all").succeeds do
-    user.credentials.all
+  tests('success') do
+    tests("#all").succeeds do
+      credentials = user.credentials.all
+      credentials.all? { |c| c.username && c.apiKey }
+    end
+
+    tests("#get").succeeds do
+      list_credential = user.credentials.all.first
+
+      credential = user.credentials.get(list_credential.identity)
+      credential.username && credential.apiKey
+    end
   end
 
-  tests("#get").succeeds do
-    credential = user.credentials.all.first
-    user.credentials.get(credential.identity)
+  tests("failure").returns(nil) do
+    user.credentials.get('i am a credential that does not exist')
   end
 end