This fixes failures in erp5_dms:testWebDavSupport.

  Traceback (innermost last):
    [...]
    Module Products.ERP5Type.patches.OFSImage, line 94, in PUT
      return PUT_orig(self, REQUEST, RESPONSE)
    Module OFS.Image, line 665, in PUT
      data = data.encode('UTF-8')
  UnicodeDecodeError: 'ascii' codec can't decode byte 0xe3 in position 10: ordinal not in range(128)