Get default values from index setting api

[krasilnikovm]

Problem:

I need to retrieve index settings like 'max_result_window' but in the response body default value is not returned, in order to fetch the default value need to add the include_default query param to /_setting request.

Suggestion:

We can add the ability to set flag include_default(or by default set to true?) and change the behavior of \Elastica\Index\Settings::get method

1. search setting value in the setting key as before
2. if the value is not found on 1 step then search setting value in default key.

[krasilnikovm]

What do think about this? is it will be useful for this lib?

[krasilnikovm]

I have prepared PR: #2115

[deguif]

Closing as this merged.