`page.date` should be undefined for non-posts

[sampsyo]

In "original" Jekyll, it seems like page.date is only defined when it comes from the filename or front matter. In gojekyll, page.date always appears to be defined—even for ordinary, non-post pages that don't have an explicit date. This date comes from the source file's mtime. I believe that seems from the final fallback case here:

gojekyll/pages/page.go

Lines 166 to 177 in f7b210e

	func (f *file) PostDate() time.Time {
	switch value := f.fm["date"].(type) {
	case time.Time:
	return value
	case string:
	t, err := evaluator.ParseDate(value)
	if err == nil {
	return t
	}
	}
	return f.modTime
	}

Do you think it might be the right thing to do here to remove the final modTime case and to return some empty value instead? If so, I can put together a PR.

(The reason this came up for me is that I use {% if page.date %} in some templates to determine whether we're rendering a post. I could presumably use some other signal instead, but I'm not sure what. :)