Added unit test for reload_object

Added test for Pry::Command::ReloadCode#reload_code in response to https://github.com/pry/pry/issues/1204

`reload_code` was actually called from another test in this file so it was technically tested. This extra test covers all of the paths and tests `reload_code` more thoroughly. It's a private method so I had to call it via `process` thus I had to do quite a bit of stubbing. Open to suggestions if anyone can suggest a cleaner way.