Keep backward compatibility

Otherwise whoever's using keys as binaries
already, which is working, would see their
code fail for "no reason"

src/hex_api_release.erl

@@ -150,13 +150,16 @@ retire(Config, Name, Version, Params0) when
     Params = maps:fold(
         fun (K, V, AccIn) ->
             AccIn#{
-                atom_to_binary(K) => V
+                maybe_atom_to_binary(K) => V
             }
         end,
         #{},
         Params0),
     hex_api:post(Config, Path, Params).
 
+maybe_atom_to_binary(K) when is_binary(K) -> K;
+maybe_atom_to_binary(K) -> atom_to_binary(K).
+
 %% @doc
 %% Unretires a package release.
 %%