put compress_level=1 inside Image._dump()

Better alternative to setting compress_level=1 than the secret handshake
in the last attempt.

The last in a series of changes to fix
https://github.com/python-pillow/Pillow/issues/2508

PIL/Image.py

@@ -592,7 +592,7 @@ class Image(object):
         else:
             if not file.endswith(format):
                 file = file + "." + format
-            self.save(file, format)
+            self.save(file, format, compress_level=1)
         return file
 
     def __eq__(self, other):