set explicit encoding when reading and writing hex config

fixes tsloughter/rebar3_hex#184

src/rebar_hex_repos.erl

@@ -163,5 +163,6 @@ update_auth_config(Updates, State) ->
     Config = auth_config(State),
     AuthConfigFile = auth_config_file(State),
     ok = filelib:ensure_dir(AuthConfigFile),
-    NewConfig = iolist_to_binary([io_lib:print(maps:merge(Config, Updates)) | ".\n"]),
-    ok = file:write_file(AuthConfigFile, NewConfig).
+    NewConfig = iolist_to_binary(["%% coding: utf-8", io_lib:nl(),
+                                  io_lib:print(maps:merge(Config, Updates)), ".", io_lib:nl()]),
+    ok = file:write_file(AuthConfigFile, NewConfig, [{encoding, utf8}]).

test/rebar_pkg_repos_SUITE.erl

@@ -9,7 +9,8 @@
 
 all() ->
     [default_repo, repo_merging, repo_replacing,
-     auth_merging, auth_config_errors, organization_merging, {group, resolve_version}].
+     auth_read_write_read, auth_merging, auth_config_errors, organization_merging,
+     {group, resolve_version}].
 
 groups() ->
     [{resolve_version, [use_first_repo_match, use_exact_with_hash, fail_repo_update,
@@ -271,6 +272,11 @@ auth_config_errors(_Config) ->
     ?assertEqual(undefined, maps:get(write_key, DefaultRepo, undefined)),
     ok.
 
+auth_read_write_read(_Config) ->
+    State = rebar_state:new([]),
+    rebar_hex_repos:update_auth_config(#{"foo" => <<200>>}, State),
+    rebar_hex_repos:update_auth_config(#{"foo" => <<200>>}, State).
+
 organization_merging(_Config) ->
     Repo1 = #{name => <<"hexpm:repo-1">>,
               api_url => <<"repo-1/api">>},