Event#append bug and behavior need clarification

I saw a weird behavior when trying to use `Event#append` to merge 2 different events.
When you try to merge two different event it will change the type of the `@timestamp` key from String to Array.

I did a quick search in our plugins and I don't see any usage of `#append`, do we really need it?

**spec to reproduce:**

``` ruby
it "should not append the timestamp" do
event = LogStash::Event.new("message" => "hello world")
event.append(LogStash::Event.new("message" => "another thing"))
expect(event.get(TIMESTAMP)).to be_kind_of(LogStash::Timestamp)
end
```

**output**

```
Failure/Error: expect(event.get(TIMESTAMP)).to be_kind_of(LogStash::Timestamp)
expected [2016-07-26T18:24:32.380Z, 2016-07-26T18:24:32.380Z] to be a kind of LogStash::Timestamp
```