fix calculate optimal part size bug

for some situations(for example, use future package), math.ceil will return an integer for python2.x,
it will cause errors for calculate optimal part size for multipart uploads. So, add __future__ module
to make sure the integer division return a float.

minio/helpers.py

@@ -25,6 +25,9 @@
 """
 
 from __future__ import absolute_import
+# if math.ceil returns an integer and devide two integers returns a float, calculate
+# part size will cause errors, so make sure division integers returns a float.
+from __future__ import division
 import io
 
 import collections

tests/unit/optimal_part_test.py

@@ -17,21 +17,27 @@
 from nose.tools import eq_, raises
 from unittest import TestCase
 
-from minio.helpers import optimal_part_info
+from minio.helpers import optimal_part_info, MAX_MULTIPART_OBJECT_SIZE
 from minio.error import InvalidArgumentError
 
 class TraceTest(TestCase):
     @raises(InvalidArgumentError)
     def test_input_size_wrong(self):
-        optimal_part_info(5000000000000000000)
+        optimal_part_info(MAX_MULTIPART_OBJECT_SIZE + 1)
 
     def test_input_size_valid_maximum(self):
-        total_parts_count, part_size, last_part_size = optimal_part_info(5497558138880)
+        total_parts_count, part_size, last_part_size = optimal_part_info(MAX_MULTIPART_OBJECT_SIZE)
         eq_(total_parts_count, 9987)
         eq_(part_size, 550502400)
         eq_(last_part_size, 241172480)
 
     def test_input_size_valid(self):
+        total_parts_count, part_size, last_part_size = optimal_part_info(MAX_MULTIPART_OBJECT_SIZE/1024)
+        eq_(total_parts_count, 1229)
+        eq_(part_size, 5242880)
+        eq_(last_part_size, 4194304)
+
+    def test_input_size_is_special_value(self):
         total_parts_count, part_size, last_part_size = optimal_part_info(-1)
         eq_(total_parts_count, 9987)
         eq_(part_size, 550502400)