Fix DataImage.ReadRangeSet()

It returns a list of one generator object, not a list
of strings.

Test: test_blockimgdiff
Change-Id: I8962c539c2ce3fae90d428b38c4b0e52c5a2cdad
diff --git a/tools/releasetools/test_blockimgdiff.py b/tools/releasetools/test_blockimgdiff.py
index 806ff4b..1aabaa2 100644
--- a/tools/releasetools/test_blockimgdiff.py
+++ b/tools/releasetools/test_blockimgdiff.py
@@ -16,7 +16,7 @@
 
 import common
 from blockimgdiff import (
-    BlockImageDiff, EmptyImage, HeapItem, ImgdiffStats, Transfer)
+    BlockImageDiff, DataImage, EmptyImage, HeapItem, ImgdiffStats, Transfer)
 from rangelib import RangeSet
 from test_utils import ReleaseToolsTestCase
 
@@ -261,3 +261,10 @@
 
     self.assertRaises(AssertionError, imgdiff_stats.Log, "/system/app/app1.apk",
                       "invalid reason")
+
+
+class DataImageTest(ReleaseToolsTestCase):
+    def test_read_range_set(self):
+        data = "file" + ('\0' * 4092)
+        image = DataImage(data)
+        self.assertEqual(data, "".join(image.ReadRangeSet(image.care_map)))