fixed bug that does not recognize the default wb resource if manually changed

I have found this bug on Stackoverflow question "http://stackoverflow.com/questions/22957776/gradle-eclipse-wtp-plugin-adding-wb-resource-removes-webappdir-from-deployment" but the documentation says 

```
      //you can add a wb-resource elements; mandatory keys: 'sourcePath', 'deployPath':
      //if sourcePath points to non-existing folder it will *not* be added.
      resource sourcePath: 'extra/resource', deployPath: 'deployment/resource'
```

But this does override all the default entries and adds only the manually added ...
IMHO this is same problem as we have had with facets, see http://forums.gradle.org/gradle/topics/additional_eclipse_wtp_facet_overwrites_default_facets, which René changed a few days ago.