xcodeproj target-diff doesn't identify differences in Bundle Resources

### Reproduction Steps

1. Create an application with a target and add some files to Copy Bundle Resources
2. Duplicate the target
3. Add some additional files to the duplicated target's Copy Bundle Resources
4. Run `xcodeproj target-diff FirstTarget SecondTarget`

### Expected Behavior

I expect to see a list of the additional files added in step 3.

### Actual Behavior

Empty output:

```
--- []
```

### Notes

Running version 1.4.4 of xcodeproj, the latest version available:

```bash
$ xcodeproj --version
1.4.4

$ gem install xcodeproj
Successfully installed xcodeproj-1.4.4
Parsing documentation for xcodeproj-1.4.4
Done installing documentation for xcodeproj after 1 seconds
1 gem installed
```