Merge pull request #796 from superhawk610/fix/config-overrides

fix: prevent unrelated `awscli` config from leaking

lib/ex_aws/config.ex

@@ -77,7 +77,7 @@ defmodule ExAws.Config do
 
       {k, v}, config ->
         case retrieve_runtime_value(v, config) do
-          %{} = result -> Map.merge(config, result)
+          %{} = result -> Map.put(config, k, result[k])
           value -> Map.put(config, k, value)
         end
     end)