[CLIENT] Meta header: support unknown faraday adapters

[derSascha]

The adapter detection in the client meta header code introduced in PR #1135 fails on unknown faraday adapters:

TypeError: no implicit conversion of nil into Hash
./lib/elasticsearch/transport/client.rb:256:in `merge'
./lib/elasticsearch/transport/client.rb:256:in `meta_header_adapter'
./lib/elasticsearch/transport/client.rb:218:in `set_meta_header'
./lib/elasticsearch/transport/client.rb:166:in `initialize'

The PR contains a fix to ignore such adapters and simply does not add any version info to the meta header.

As a workaround until fix has been released: The meta header can be disabled using enable_meta_header: false in the client options.

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[picandocodigo]

@derSascha thank you very much for this contribution!

I'll backport it and publish a patch release with the fix 👍