Fix invalid headers

[kenkendk]

This fixes a bug introduced with 6be6ef0.

Bad news is that all zip files written with 0.15.1 are semi-broken.
That is, their filenames are garbled, but content is recoverable.

[kenkendk]

As far as I can see the code here is not used, so my panic was unjustified.

For some reason, the ZipHeader has an abstract Write method, and all the subclasses implement it, but none of them call it.
This would only cause problems if someone were to manually construct the items and call the write methods.