Remove the capital from our custom header name when running in wasm

[irevoire]

Pull Request

Related issue

Fixes https://github.com/meilisearch/meilisearch-rust/issues/580

[minijackson]

I can confirm that my setup works on this branch. Thanks!

[meili-bors[bot]]

Build succeeded:

• clippy-check
• integration-tests
• readme-check
• rust-format
• wasm-build
• Yaml linting check

[irevoire]

Thanks @minijackson, perfect