Event clone, performs shallow copy, breaking cloned events.

In logstash 2.3.x
- When cloning events, with depth > 2, changes to the cloned event will modify the original event.

In the folowing test, the original event, in message should be unchanged, but after the mutate, applied to the cloned event, the original event, gets modified.

``` ruby
input {
generator {
message => '{"a": {"a" : "v aa"} ,"b": {"a":{"a":"v b_aa","b":"v bab"}}}'
codec => json
count => 1
}
}

filter {
clone {
clones => ["clone"]
}

if [type] == "clone" {
mutate {
rename => {
"[a][a]" => "[c_aa]"
"[b][a][a]" => "[c_ba][a]"
# "[b][a][b]" => "[c_ba][b]"
}
}
}
}

output {
stdout { codec => rubydebug { metadata => true } }
}
```

[b][a][a] gets removed from the original event due to the later mutate in the cloned event.

The issue seems to be identified in the 2.3 code event.rb.

``` ruby
# Create a deep-ish copy of this event.
def clone
copy = {}
@data.each do |k,v|
#TODO(sissel): Recurse if this is a hash/array?
copy[k] = begin v.clone rescue v end
end

self.class.new(copy)
end
```