Index should bulk store to its own URL endpoint (fixes #327)

[nz]

The Index#bulk_import method (index.rb:123), and other index methods, interact with the Cluster URL rather than their own Index URL endpoint.

This pull request refactors the index URL endpoint into the Index#url getter method, refactors a number of methods to use this new getter for clarity, and changes the bulk_import method to use this index-level endpoint rather than the cluster-level URL endpoint.

Also updated/refactored test expectations accordingly.

This fixes issues (e.g., #327) for users trying to configure Tire to use a single application index and who do not have access to cluster-level methods like /_bulk in production.

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If anyone following along wants to test, try this in your Gemfile:

gem 'tire', git: 'git@github.com:nz/tire', branch: 'index-bulk-import-to-index-endpoint'

[nz]

thanks! ❤️