Fix metadata testing failures

This bug was introduced in the recent metadata patch. The problem was
caused by a testing-specific monkeypatch on LogStash::Event#[]=

I fixed this by moving the strict_set input validation from
LogStash::Util::Accessors to LogStash::Event as a class method.
Then monkeypatched the Event#[]=  to invoke validation before doing
the set operation. This now makes it call the original []= method
and should help keep future breakages from happening.

Fixes #1936