This header is optional. From 
http://www.pkware.com/documents/casestudies/APPNOTE.TXT

      Although not originally assigned a signature, the value 
      0x08074b50 has commonly been adopted as a signature value 
      for the data descriptor record.  Implementers should be 
      aware that ZIP files may be encountered with or without this 
      signature marking data descriptors and should account for
      either case when reading ZIP files to ensure compatibility.
      When writing ZIP files, it is recommended to include the
      signature value marking the data descriptor record.  When
      the signature is used, the fields currently defined for
      the data descriptor record will immediately follow the
      signature.

So this is Not A Bug (and "zip -T" correctly reports a Warning)

However, I suggest to follow the recommendation and add the signature, 
for 2.7/3.1 only, and not to backport.