[Elixir] Use string headers

[ntodd]

Currently, generated API modules set headers with the following function:

request
|> add_param(:headers, :"Foo-Id", foo_id)

This works well with the default Tesla adapter, but when changing the adapter to Finch, I got errors that Mint was trying to call String.downcase/2 on the atom key causing a FunctionClauseError.

Additionally, Tesla expects headers as a keyword list of binaries (e.g. [{"Foo-Id", "1234"}] not [{"Foo-Id", 1234}]. If a non-string header value is provided, Mint.HTTP1.Request.validate_header_value/2 will throw :invalid_header_value. Again, this was only observed when changing the default adapter, but does follow the spec expected by Tesla.put_headers/2 so it should probably be changed regardless.

This PR does two things:

1. Changes the add_param/4 function to use "Foo-Id" rather than :"Foo-Id"
2. Calls Kernel.to_string/1 on the header value to ensure it is a binary value

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@mrmstn

[mrmstn]

Hi @ntodd,

Oh, I wasn’t aware of that - thank you very much for the PR and for pointing it out! The changes look good to me, although I’d like to run a quick test in the wild just to make sure it won’t cause any issues with the default adapter. If everything checks out, we should be good to merge this PR by tomorrow.

Thanks again!

[mrmstn]

Hi @ntodd

Your proposed changes look good, but I noticed that this won't affect how optional parameters are handled. Those values are still stored as atoms.

From the implementation perspective, I think it would make more sense to only change the function add_param(request, :headers, key, value)
as this would ensure that the key will always be an atom, until passed on to the actual transport - and this should also work for the optional parameters as headers

What do you think?

[ntodd]

@mrmstn I have no problem with this, but I won't have a chance to tackle it for a while. I'd be happy to review and verify, however.

[ntodd]

@mrmstn I updated add_param/4 as suggested. Please review and let me know if you have any remaining issues.