Use List.keyfind/3 instead of Access protocol to retrieve header values

Fixes aws-beam/aws-elixir#5

Author: jcomellas

priv/rest_json.ex.eex

@@ -24,9 +24,9 @@ defmodule <%= context.module_name %> do
             {"<%= parameter.location_name %>", "<%= parameter.name %>"},<% end %>
           ]
           |> Enum.reduce(body, fn {header_name, key}, acc ->
-            case response.headers[header_name] do
+            case List.keyfind(response.headers, header_name, 0) do
               nil -> acc
-              value -> Map.put(acc, key, value)
+              {_header_name, value} -> Map.put(acc, key, value)
             end
           end)
 
@@ -54,9 +54,9 @@ defmodule <%= context.module_name %> do
             {"<%= parameter.location_name %>", "<%= parameter.name %>"},<% end %>
           ]
           |> Enum.reduce(body, fn {header_name, key}, acc ->
-            case response.headers[header_name] do
+            case List.keyfind(response.headers, header_name, 0) do
               nil -> acc
-              value -> Map.put(acc, key, value)
+              {_header_name, value} -> Map.put(acc, key, value)
             end
           end)