generate method (optionally) saves file

This way, there's a creation counterpart to `delete()`. The user
shouldn't have to deal with storage backends to create and delete the
files, and now they don't.

imagekit/cachestate/__init__.py

@@ -16,9 +16,7 @@ class DefaultCacheStateBackend(object):
 
         """
         if self.is_invalid(file):
-            content = file.generate()
-            if content:
-                file.storage.save(file.name, content)
+            file.generate(save=True)
 
     def invalidate(self, file):
         file.delete(save=False)

imagekit/models.py

@@ -240,7 +240,7 @@ class ImageSpecFile(_ImageSpecFileMixin, ImageFieldFile):
     def validate(self):
         return self.field.cache_state_backend.validate(self)
 
-    def generate(self):
+    def generate(self, save=True):
         """
         Generates a new image file by processing the source file and returns
         the content of the result, ready for saving.
@@ -257,6 +257,10 @@ class ImageSpecFile(_ImageSpecFileMixin, ImageFieldFile):
             fp = StringIO(fp.read())
 
             img, content = self._process_content(self.name, fp)
+
+            if save:
+                self.storage.save(self.name, content)
+
             return content
 
     @property