Avoid Array instances when JSON'ifying Arrays / Hashes

Similar to the previous commit, we avoid the unnecessary allocation of
a temporary array when looping over the `@raw` value. In addition to
that, we ensure thread-safety by always operating on a copy of the raw
data so that other threads can not change anything under our feet
anymore.

lib/rackstash/fields/array.rb

@@ -89,7 +89,7 @@ module Rackstash
 
       # @return [::Array] deep-transforms the array into a plain Ruby Array
       def as_json(*)
-        @raw.map { |value|
+        @raw.to_a.map! { |value|
           value.is_a?(AbstractCollection) ? value.as_json : value
         }
       end

lib/rackstash/fields/hash.rb

@@ -54,9 +54,9 @@ module Rackstash
 
       # @return [::Hash] deep-transforms the hash into a plain Ruby Hash
       def as_json(*)
-        @raw.each_with_object({}) do |(key, value), memo|
-          value = value.as_json if value.is_a?(AbstractCollection)
-          memo[key] = value
+        hash = @raw.to_h
+        hash.each_pair do |key, value|
+          hash[key] = value.as_json if value.is_a?(AbstractCollection)
         end
       end
       alias to_hash as_json