Don't add extraneous metadata padding

If the metadata field is already aligned to a 16-byte boundary then
don't add a additional padding.  This allows the user to create a
16-byte aligned extension padded with 0x20 that can be read by the Jim
software (which doesn't like finding 0x00 in the metadata chunk).

Author: jstutters

nibabel/nifti1.py

@@ -334,7 +334,8 @@ def get_sizeondisk(self):
         size = len(self._mangle(self._content))
         size += 8
         # extensions size has to be a multiple of 16 bytes
-        size += 16 - (size % 16)
+        if size % 16 != 0:
+            size += 16 - (size % 16)
         return size
 
     def __repr__(self):