Fix bug LOGSTASH-1225 "Cannot Clone Fixnum"

The clone {} filter will throw an exception if any of the values in the
event map are numbers. Problem is that numbers are not clonable.

This is a single-line fix that wraps the .clone call in a try block and
does a simple set-by-value if the clone raises an exception.

Included under spec/filters/clone.rb is a test case that fails without
the change in lib/logstash/event.rb and passes with the fix applied.