add failing test for minimal POST

POST with only 'type' key is not triggering a save on the activerecord model, but no errors are reported. This leads to an incorrect 201 status.

Author: adamworrall

test/integration/requests/request_test.rb

@@ -156,6 +156,17 @@ def test_post_single
     assert_equal 201, status
   end
 
+  def test_post_single_minimal_invalid
+    post '/posts',
+      {
+        'data' => {
+          'type' => 'posts'
+        }
+      }.to_json, "CONTENT_TYPE" => JSONAPI::MEDIA_TYPE
+
+    assert_equal 422, status
+  end
+
   def test_update_association_without_content_type
     ruby = Section.find_by(name: 'ruby')
     patch '/posts/3/links/section', { 'data' => {type: 'sections', id: ruby.id.to_s }}.to_json
@@ -189,7 +200,7 @@ def test_patch_update_association_has_many_acts_as_set
   def test_post_update_association_has_many
     rogue = Comment.find_by(body: 'Rogue Comment Here')
     post '/posts/5/links/comments', { 'data' => [{type: 'comments', id: rogue.id.to_s }]}.to_json, "CONTENT_TYPE" => JSONAPI::MEDIA_TYPE
-    
+
     assert_equal 204, status
   end