Fix failure cases with integration test

[tungleduyxyz]

I found that we have a strange API: https://killbill.github.io/slate/invoice.html#retrieve-the-manualpay-invoice-template-for-the-tenant
This API url will work:
http://localhost:8080/1.0/kb/invoices/manualPayTemplate/
but this one will not work:
http://localhost:8080/1.0/kb/invoices/manualPayTemplate
it’s because of we have an option with language for example: http://localhost:8080/1.0/kb/invoices/manualPayTemplate/en
So I made an default option for the client API: https://github.com/killbill/killbill-client-ruby/pull/115/files

[Copilot]

Pull Request Overview

This PR addresses failures in integration tests by ensuring a default locale is applied and improving parameter encoding compatibility.

• Adds a default 'en' locale fallback in invoice template retrieval to prevent nil-related errors.
• Replaces filter_map with map + compact in HTTP parameter encoding for broader Ruby version support.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lib/killbill_client/models/invoice.rb	Introduce `locale
lib/killbill_client/api/net_http_adapter.rb	Change parameter enumeration from filter_map to map + compact

Comments suppressed due to low confidence (4)

lib/killbill_client/models/invoice.rb:215

• Consider updating the method’s documentation or docstring to mention that locale now defaults to 'en', so callers are aware of this behavior change.

          locale ||= 'en'

lib/killbill_client/models/invoice.rb:215

• Add or update integration/unit tests to verify that omitting locale results in using 'en' and that the correct endpoint is called.

          locale ||= 'en'

lib/killbill_client/api/net_http_adapter.rb:160

• Add tests for encode_params covering cases with nil values, arrays, and the withStackTrace flag to ensure encoding changes behave as expected.

          pairs = options[:params].map { |key, value|

lib/killbill_client/api/net_http_adapter.rb:160

• [nitpick] Using map + compact incurs two iterations; consider using filter_map if targeting Ruby ≥ 2.7 to perform filtering and mapping in one pass.

          pairs = options[:params].map { |key, value|