Fix for handling accessor sets on array elements.

Previously: Filters like mutate would throw an exception when attempting
to convert an element within an array to a specific type. This was
raising an error because the index in the field reference was being
treated as a String instead of an integer representing the index of the
field to convert.

Now: Plugins like Mutate can access and convert elements within arrays
to specific types, or, updating their values.

related issue: https://github.com/elasticsearch/logstash/issues/1401
I would prefer more

```ruby
target[target.is_a?(Array) ? key.to_i : key] = value
```
or less

```ruby
key = key.to_i if target.is_a?(Array)
target[key] = value
```