Use http.StatusConflict instead of hard coded value in SubmitChange

[opalmer]

The use of http.StatusConflict does not change how the code functions, it just makes it easier to read.

[coveralls]

Coverage remained the same at 10.495% when pulling a4121caf7f8ee8e3bcb56268c6ad1bca5ca65c5d on cleanup_status_code into b7d35b49dd8d5ab3872cffc5da834f7a1c34946c on master.

[andygrunwald]

@opalmer is on 🔥