Add breaking test

See #245 for context.

The TL;DR is the response sometimes returns a String instead of Hash object. This should be handled properly.

The way to handle this is up for discussion. To start addressing the issue the String response will just be surfaced as `latest_version`.

Author: vietqhoang

test/npm_test.rb

@@ -94,4 +94,17 @@ def code() "200" end
       end
     end
   end
+
+  test "return response is a String type" do
+    response = "version not found".to_json
+
+    @npm.stub(:get_json, response) do
+      outdated_packages = @npm.outdated_packages
+
+      assert_equal(1, outdated_packages.size)
+      assert_equal('md5', outdated_packages[0].name)
+      assert_equal('2.2.0', outdated_packages[0].current_version)
+      assert_equal('version not found', outdated_packages[0].latest_version)
+    end
+  end
 end