Update library/Zend/Feed/Writer/Entry.php

Strings are not acceptable types for dates. Matches the behaviour of the AbstractFeed class for setting dates there.
Merge pull request #3762 branch 'hotfix/cs'
Merge commit 'refs/pull/3761/head' of github.com:zendframework/zf2 into hotfix/fix-docblock-type
Forward #3761