Check for Entry status instead of published bool

[joshuablum]

Fixes https://github.com/pecotamic/sitemap/issues/22.

This small tweak leaves Entries out that are not supposed to be publicly accessible.

Please test it properly. It was just me doing a quick change.

[werner-freytag]

Thanks for your bug submission and your fix, @joshuablum! I've checked it, it works as intended.