Fix crash on conflict

This ***may*** fix issues #191 and #192. 

[Someone on the mailing list](http://groups.yahoo.com/neo/groups/todotxt/conversations/topics/5255) mentioned that renaming their 'Todo' directory to 'todo' fixed the issue, reminding me of [this issue](https://github.com/ginatrapani/todo.txt-android/pull/283) we had with the Android app. Changing my directory to 'Todo' made the app think there was a conflict, then it crashed in the simulator while attempting to display the conflict error. 

So, there were two fixes here:

1. Do a case-insensitive comparison when checking for a conflict, just like we had to do for the Android app.
2. Fix the crash by passing a string to the method that was expecting one, instead of a DropboxFile object.