Logstash: do not use `date` on json event @timestamp

When the json codec is used to create an event, logstash automatically
parses the "@timestamp" member. Attempting to further process this field
with the `date` filter results in warnings such as "cannot convert
instance of class org.jruby.RubyTime to class java.lang.String".

Change-Id: I84cf3b394d0c8e4a2c866ea1d02a4462525ca960