Writer: append trailing slash to <= 15-byte GNU file names (blakesmith#14)

The GNU variant of the ar format expects all real file names to have
trailing slashes, regardless of whether they are stored in the string
table or in the file name header field.

Author: chrisnovakovic

writer.go

@@ -216,6 +216,10 @@ func (aw *Writer) WriteHeader(hdr *Header) error {
 			}
 			aw.string(s.next(16), "/"+strconv.Itoa(offset))
 		} else {
+			// File names beginning with "/" aren't real file names - don't append "/" to them.
+			if hdr.Name[0] != '/' {
+				hdr.Name = hdr.Name + "/"
+			}
 			aw.string(s.next(16), hdr.Name)
 		}
 	case BSD: