Dry run fixes

I had an issue with the 'DisablePreprocessImageData' when running the importer in dry run mode: https://github.com/ho-nl/Ho_Import/issues/42
This pull request fixes this.
Other fixes:
1. I saw there was an inconsistency between `setIsDryrun` and `getIsDryRun` (capital 'R')
2. There were some other inconsistencies between the normal import and dry run import methods, now they should be equal (maybe you should consider moving that code into a separate method, as it is almost identical code)
3. There was an issue with setting the default value for 'DisablePreprocessImageData' in the constructor, this is also fixed.

Watch out: I didn't test any of these changes, so I hope everything is ok?