Fix index settings being wiped

[epbarger]

• Fixes https://github.com/algolia/gatsby-plugin-algolia/issues/110, where existing index settings are completely replaced on indexing, even if you don't pass any settings in. Now your existing settings will only be replaced if you provide them.
• Adds note to the example in the README to clarify behavior.

I manually tested providing settings and not providing settings to make sure the behavior is correct now. I did not test anything around replicas, but I don't see any reason why that would be affected. From what I can tell the replicas feature currently only works if you provide settings in the plugin, because of the reliance on settings.replicaUpdateMode.

[urre]

Hey @Haroenv @epbarger I have this same problem where the facet settings, searchable attributes get wiped when running npm run build. Can I test this PR in some way in my site? Or if you have other suggestions? I tested to remove the settings object in my queries array in my algolia script, but that didn't help

[Haroenv]

if you take the file in your node_modules and edit it to this version, you can test it @urre

[urre]

@Haroenv ah, I'll test

[epbarger]

Sorry I've been swamped with other work and haven't gotten back to this yet. If either one of you is able to take it over the finish line, please feel free!

[urre]

@epbarger it seemed to work, I tested it in my node_modules folder. Is it much left to do to get this published? We are going to publish our search this week, and would be awesome if this fix could be installed from npm

[Haroenv]

Thanks both @urre and @epbarger, I've released this as 0.16.3!

[urre]

Thanks!