Panic while generating YNAB OpenAPI client

Hi there!

I tried to generate a client for the YNAB OpenAPI spec which seems to panic:

❯ cargo install cargo-progenitor
    Updating crates.io index
     Ignored package `cargo-progenitor v0.4.1` is already installed, use --force to override

❯ RUST_BACKTRACE=1 cargo progenitor --input ynab_openapi_spec.yaml --output ynab --name ynab --v
ersion 0.1.0
thread 'main' panicked at /Users/opeik/.cargo/registry/src/index.crates.io-6f17d22bba15001f/progenitor-impl-0.4.1/src/method.rs:1201:9:
assertion failed: response_types.len() <= 1
stack backtrace:
   0: rust_begin_unwind
             at /rustc/a2d9d73e608f1b24eba840c4fd2d68dbe3b65e01/library/std/src/panicking.rs:647:5
   1: core::panicking::panic_fmt
             at /rustc/a2d9d73e608f1b24eba840c4fd2d68dbe3b65e01/library/core/src/panicking.rs:72:14
   2: core::panicking::panic
             at /rustc/a2d9d73e608f1b24eba840c4fd2d68dbe3b65e01/library/core/src/panicking.rs:144:5
   3: progenitor_impl::method::<impl progenitor_impl::Generator>::extract_responses
   4: progenitor_impl::method::<impl progenitor_impl::Generator>::method_sig_body
   5: progenitor_impl::method::<impl progenitor_impl::Generator>::positional_method
   6: <alloc::vec::Vec<T> as alloc::vec::spec_from_iter::SpecFromIter<T,I>>::from_iter
   7: core::iter::adapters::try_process
   8: progenitor_impl::Generator::generate_tokens
   9: cargo_progenitor::main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

Reproduce the panic with cargo progenitor using the YNAB OpenAPI spec linked in the issue and the shown command. Start by examining extract_responses in progenitor-impl/src/method.rs around the response_types assertion. Done means the specification no longer causes a panic and client generation completes successfully.