Fix: single-element array params not being passed as array

[jfpimentel]

Hey @anttiviljami,

This PR fixes https://github.com/anttiviljami/openapi-client-axios/issues/111.

I wrote a test to reproduce the issue and it's passing with these changes.

I'm not sure if this is the right way to fix it, but let me know what you think.

Thanks, Joao

[anttiviljami]

LGTM ✅