Fix request_signer body serialization and wildcard double-encoding

[xluo-aws]

Description

Fixes two bugs that should be resolved before the 4.0.0 release.

Fix 1 — request_signer body serialization mismatch (#311)

The `request_signer` feature (added in 4.0 beta via #281) was passing the raw Ruby `body` object (a Hash) to `sign_request`. However, the Faraday transport then serialized the body separately via `__convert_to_json`. When these two serializations produced different byte strings (different key order, custom serializer, etc.), the resulting signature didn't match the wire body, causing 403 errors on any signed request with a body.

Fix: serialize the body through the transport's own serializer before passing it to `sign_request`, mirroring exactly what the transport sends on the wire. String bodies are passed through unchanged.

Fix 2 — wildcard/special character double-encoding in index params (#319)

`normalize_value` used `CGI.escape` which encodes `*` → `%2A`. The HTTP layer then percent-encodes the `%` → `%25`, so OpenSearch received `test-%2A` literally instead of treating it as a wildcard, causing `index_not_found_exception` errors.

Fix: switch to `URI::DEFAULT_PARSER.escape` with a safe-character set that preserves `*` and other characters meaningful in OpenSearch path patterns. Spaces are encoded as `%20` (correct for URL paths) rather than `+`.

Issues Resolved

Fixes #311
Fixes #319

Testing

• Updated `request_signer_spec.rb`: existing test updated to reflect correct serialized-string behaviour; 3 new cases covering Hash body, nil body, and pre-serialized String body.
• Updated `utils_spec.rb`: existing expectation updated for new `%20` encoding; 6 new `#normalize_value` cases covering wildcard preservation, comma-separated patterns, special chars, and arrays.
• Full unit suite: 22 RSpec examples + 116 minitest runs + 107 transport tests — 0 failures.

Check List

• New functionality includes testing.
• New functionality has been documented (CHANGELOG).
• Commits are signed per the DCO using `--signoff`.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.