Fix AttributeError: class Image has no attribute 'DEBUG'

For example:
```
======================================================================
ERROR: test_blur (test_file_libtiff.TestFileLibTiff)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "D:\Build\Pillow\Pillow-git\Tests\test_file_libtiff.py", line 263, in test_blur
    im.close()
  File "D:\Build\Pillow\Pillow-git\PIL\Image.py", line 542, in close
    if Image.DEBUG:
AttributeError: class Image has no attribute 'DEBUG'
```