[Elixir] Use string headers (#19759)

* Use string headers in Elixir request builder

* Add samples

* Update add_param spec

* Updated samples with spec changes

* Convert header values and keys to strings in add_param/4

Author: ntodd

modules/openapi-generator/src/main/resources/elixir/request_builder.ex.mustache

@@ -104,7 +104,7 @@ defmodule {{moduleName}}.RequestBuilder do
     headers =
       request
       |> Map.get(:headers, [])
-      |> List.keystore(key, 0, {key, value})
+      |> List.keystore(to_string(key), 0, {to_string(key), to_string(value)})
 
     Map.put(request, :headers, headers)
   end

samples/client/petstore/elixir/lib/openapi_petstore/request_builder.ex

@@ -106,7 +106,7 @@ defmodule OpenapiPetstore.RequestBuilder do
     headers =
       request
       |> Map.get(:headers, [])
-      |> List.keystore(key, 0, {key, value})
+      |> List.keystore(to_string(key), 0, {to_string(key), to_string(value)})
 
     Map.put(request, :headers, headers)
   end