Ensure the squashed estimated size is an int

Also adds a test to verify

formats/squashed.py

@@ -5,6 +5,7 @@ from formats.tarimageformatter import TarImageFormatter
 
 import copy
 import json
+import math
 
 class FileEstimationException(Exception):
   """ Exception raised by build_docker_load_stream if the estimated size of the layer TAR
@@ -68,6 +69,9 @@ class SquashedDockerImage(TarImageFormatter):
         image_json = get_image_json(image)
         estimated_file_size += image_json.get('Size', 0) * SquashedDockerImage.SIZE_MULTIPLIER
 
+    # Make sure the estimated file size is an integer number of bytes.
+    estimated_file_size = int(math.ceil(estimated_file_size))
+
     yield self.tar_file_header(synthetic_image_id + '/layer.tar', estimated_file_size)
 
     # Yield the contents of the merged layer.