Cover diff_positionable_note JSON issue

Adds spec coverage and a fix for handling of position fields
once the `json` gem upgrade has taken place.

app/models/concerns/diff_positionable_note.rb

@@ -23,6 +23,8 @@ module DiffPositionableNote
       if new_position.is_a?(Hash)
         new_position = new_position.with_indifferent_access
         new_position = Gitlab::Diff::Position.new(new_position)
+      elsif !new_position.is_a?(Gitlab::Diff::Position)
+        new_position = nil
       end
 
       return if new_position == read_attribute(meth)

spec/support/shared_examples/models/diff_positionable_note_shared_examples.rb

@@ -49,5 +49,29 @@ RSpec.shared_examples 'a valid diff positionable note' do |factory_on_commit|
         expect(subject.errors).to have_key(:commit_id)
       end
     end
+
+    %i(original_position position change_position).each do |method|
+      describe "#{method}=" do
+        it "doesn't accept non-hash JSON passed as a string" do
+          subject.send(:"#{method}=", "true")
+          expect(subject.attributes_before_type_cast[method.to_s]).to be(nil)
+        end
+
+        it "does accept a position hash as a string" do
+          subject.send(:"#{method}=", position.to_json)
+          expect(subject.position).to eq(position)
+        end
+
+        it "doesn't accept an array" do
+          subject.send(:"#{method}=", ["test"])
+          expect(subject.attributes_before_type_cast[method.to_s]).to be(nil)
+        end
+
+        it "does accept a hash" do
+          subject.send(:"#{method}=", position.to_h)
+          expect(subject.position).to eq(position)
+        end
+      end
+    end
   end
 end