allows integration with openapispex

[juantascon]

some tools like https://github.com/open-api-spex/open_api_spex convert Conn.params into a custom struct, this change allows to integrate with this type of tools by converting the struct back into a map

[navinpeiris]

Thanks @juantascon! It'll be awesome if we can include a simple test for this as well, just so that we don't break it in the future

[juantascon]

@navinpeiris done, please review again

[navinpeiris]

Thanks @juantascon!