[OskarEichler]

Summary

Preserve false bodies so JSON middleware can encode them.

Reproduction and verification

With request :json and the test adapter echoing env.body, post('/', false) produces an empty string before the fix and 'false' afterward. Verified POST/PUT/PATCH, true/zero/hash/array/string bodies and block overrides.

• Individual patch: 21 focused Faraday checks passed (body); all 639 existing RSpec examples pass; changed-file RuboCop and Ruby syntax pass.
• Combined installed-release-based branch: 639 existing examples, 1,124 focused checks, full RuboCop (75 files), YARD checks and gem packaging pass.
• External faraday-net_http adapter suite: 386 existing examples, zero failures with the combined fixes.
• Ruby 4.0.6 through rbenv. No production or live external HTTP operations.

Limitations

No test/spec files were added or changed because the originating repository explicitly forbids this. Reproductions ran outside the repository. This differs from Faraday's requested regression-test contribution convention and is disclosed for maintainer review. Other Ruby versions/platforms were not run locally. This PR includes only the focused source change; the other fixes and the consumer's separately verified merged JSON decoder patch #1687 are not added here.

Breaking-change notes

False now reaches middleware instead of being silently discarded. Nil behavior is unchanged.

[iMacTia]

I checked with a JSON validator and false is indeed a valid JSON body, so this change makes sense. I just want to add a small test case to make sure this and other literals are correctly covered and tested

[iMacTia]

Tests added in #1728, the PR CI is currently failing exactly on the false scenario, as expected.

This PR fixes that 👍