BF+TST: Python 3 zero bytes error for offset

Nipy testing revealed that when the data offset is beyond the end of the
header we hit the unguarded use of '\x00' for a zero byte string, rather
than the needed ZEROB constant.

Author: matthew-brett

nibabel/nifti1.py

@@ -1779,7 +1779,7 @@ def _write_header(self, header_file, header, slope, inter):
         offset = header.get_data_offset()
         diff = offset-header_file.tell()
         if diff > 0:
-            header_file.write('\x00' * diff)
+            header_file.write(ZEROB * diff)
 
     def update_header(self):
         ''' Harmonize header with image data and affine '''

nibabel/tests/test_nifti1.py

@@ -469,6 +469,15 @@ def test_set_sform(self):
         # Unexpected keyword raises error
         assert_raises(TypeError, img.get_sform, strange=True)
 
+    def test_hdr_diff(self):
+        # Check an offset beyond data does not raise an error
+        img = self.image_class(np.zeros((2,3,4)), np.eye(4))
+        ext = dict(img.files_types)['image']
+        hdr = img.get_header()
+        hdr['vox_offset'] = 400
+        with InTemporaryDirectory():
+            img.to_filename('another_file' + ext)
+
 
 class TestNifti1Pair(TestNifti1Image):
     # Run analyze-flavor spatialimage tests