fix: json >= 3.0 is causing a wrong number of arguments

[dominikschidlowskidmde]

Summary

Fixes #426. json v3.0 changed JSON.parse's signature to keyword-only
arguments and removed the create_additions option entirely, so Client#success
and Client#error raise ArgumentError: wrong number of arguments (given 2, expected 1) on any successful or JSON-error response once json 3.x is
resolved.

Changes

• JSON.parse(response.body, JSON_PARSE_OPTIONS) → JSON.parse(response.body, **JSON_PARSE_OPTIONS)
  at the two call sites in lib/vault/client.rb.
• Dropped create_additions: false from JSON_PARSE_OPTIONS — json 3.x
  removed the option outright. Not a behavior change: create_additions has
  defaulted to false for JSON.parse (as opposed to JSON.load) since at
  least v1.8.6.
• Added unit coverage in spec/unit/client_spec.rb exercising #success/#error
  against a real JSON response body — the existing suite passed unchanged
  under json 3.0.2 even without this fix, since nothing drove a JSON-typed
  response through those methods.

[hashicorp-cla-app]

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