update accessors#get does not mutate @store

This fixes Accessors so that when one `get`s a
field reference, it does not inject an empty path
into the original store

before, there were side-effects:
data = {}
a = LogStash::Util::Accessors.new(data)
a.get("[hello][world]")
//(data mutates) data == {"hello" => {}}

Fixes #2294