Test that Optimistic strategy doesn't cause reads

Using the Optimistic strategy should prevent IO ops when you cast the
file as a boolean.

setup.py

@@ -48,6 +48,7 @@ setup(
         'nose-progressive==1.5',
         'django-nose==1.2',
         'Pillow<3.0',
+        'mock==1.0.1',
     ],
     test_suite='testrunner.run_tests',
     install_requires=[

tests/test_optimistic_strategy.py

@@ -0,0 +1,37 @@
+from nose.tools import assert_true, assert_false
+from imagekit.cachefiles import ImageCacheFile
+from mock import Mock
+from .utils import create_image
+from django.core.files.storage import FileSystemStorage
+from imagekit.cachefiles.backends import Simple as SimpleCFBackend
+from imagekit.cachefiles.strategies import Optimistic as OptimisticStrategy
+
+
+class ImageGenerator(object):
+    def generate(self):
+        return create_image()
+
+    def get_hash(self):
+        return 'abc123'
+
+
+def get_image_cache_file():
+    storage = Mock(FileSystemStorage)
+    backend = SimpleCFBackend()
+    strategy = OptimisticStrategy()
+    generator = ImageGenerator()
+    return ImageCacheFile(generator, storage=storage,
+                          cachefile_backend=backend,
+                          cachefile_strategy=strategy)
+
+
+def test_no_io_on_bool():
+    """
+    When checking the truthiness of an ImageCacheFile, the storage shouldn't
+    peform IO operations.
+
+    """
+    file = get_image_cache_file()
+    bool(file)
+    assert_false(file.storage.exists.called)
+    assert_false(file.storage.open.called)