Patch #651621, approved by MvL.

This patch allows ZipFile.writestr() to be called with
an archive file name instead of a ZipInfo instance:

z = ZipFile("myarchive.zip", "w")
z.writestr("foo/baz/file.ext", data)
z.close()

I found the old writestr() method very inconvenient
for simple (but common) things.

If called with a file name instead of a ZipInfo
instance, the date_time is set to the current date/time,
which makes sense to me for anonymous data.