added test for submitting with the wrong content type header

spec/create_spec.rb

@@ -88,4 +88,14 @@ describe "Submit API request to create new link" do
     response_json = JSON.parse(last_response.body)
     expect(response_json['message']).to eq('url does not resolve')
   end
+
+  it "should return with a 400 status and 'not a valid json request' message when a request is made with the wrong content type header" do
+    data = {
+      url: 'http://google.com'
+    }
+    post('/links', data.to_json)
+    expect(last_response.status).to eq(400)
+    response_json = JSON.parse(last_response.body)
+    expect(response_json['message']).to eq('not a valid json request')
+  end
 end