ServeContent ignored zero time.Time{} values when generating
Last-Modified response headers and checking If-Modified-Since request
headers. Do the same for a time.Time representing the Unix epoch zero
value, as this is a common bogus value. Callers who really want to
send that value (incredibly unlikely) can add a nanosecond to it and
it will be truncated to second granularity anyway.

Fixes #9842

Change-Id: I69f697bfc4017404a92a34e3fe57e2711c1e299d
Reviewed-on: https://go-review.googlesource.com/7915Reviewed-by: David Symonds <dsymonds@golang.org>