BATCH-1957: Add StaxEventItemWriter deleteIfEmpty property
add unit test test showing bug in FlatFileItemWriter: output file is
deleted on restart if shouldDeleteIfEmpty property is set and no new
records have been written
add additional test case that shows a bug in the current
StaxEventItemWriter.shouldDeleteIfEmpty: when the writer restarts from a
previous execution where the output file was deleted, this fails because
FileUtils.setUpOutputFile expects the file to be there on restart
- restore WRITE_STATISTICS_NAME from execution context on restart
- don't assume output file is always present on restart (could be
deleted by previous execution if shouldDeleteIfEmpty flag was set)
BATCH-1957: Updated comments
Merge pull request 134 from jpraet/BATCH-1957

* BATCH-1957:
  BATCH-1957: Updated comments
  - restore WRITE_STATISTICS_NAME from execution context on restart - don't assume output file is always present on restart (could be deleted by previous execution if shouldDeleteIfEmpty flag was set)
  add additional test case that shows a bug in the current StaxEventItemWriter.shouldDeleteIfEmpty: when the writer restarts from a previous execution where the output file was deleted, this fails because FileUtils.setUpOutputFile expects the file to be there on restart
  add unit test test showing bug in FlatFileItemWriter: output file is deleted on restart if shouldDeleteIfEmpty property is set and no new records have been written
  BATCH-1957: Add StaxEventItemWriter deleteIfEmpty property