archive/zip: FileHeader.FileInfo().ModTime() should use Modified

[Xjs]

Since Go 1.10, archive/zip.FileHeader contains a Modified field supporting extended timestamps. However, FileHeader.FileInfo() returns a os.FileInfo implementation whose ModTime() method still returns the legacy mod time. This should be changed to return Modified. A hint documenting that FileInfo() uses legacy fields would be nice for the time being.

As far as I can tell, headerFileInfo has direct access to the Modified field. Is there a reason why it wasn't included that I am missing, or was it just an oversight?

[gopherbot]

Change https://golang.org/cl/144382 mentions this issue: archive/zip: use Modified in FileHeader.FileInfo()

[mattn]

As far as I remember, this is discussed in #18359

[bcmills]

CC @dsnet @bradfitz

[dsnet]

Already commented on the CL. I believe the change is in the right direction, but needs to have a fallback for the legacy timestamp fields.