[rubygems/rubygems] Always join with a string

Gem.path_separator can also be a regexp...

https://github.com/rubygems/rubygems/commit/7e6e56a5f3

lib/rubygems/commands/exec_command.rb

@@ -56,7 +56,7 @@ to the same gem path as user-installed gems.
   end
 
   def execute
-    gem_paths = { "GEM_HOME" => Gem.paths.home, "GEM_PATH" => Gem.paths.path.join(Gem.path_separator), "GEM_SPEC_CACHE" => Gem.paths.spec_cache_dir }.compact
+    gem_paths = { "GEM_HOME" => Gem.paths.home, "GEM_PATH" => Gem.paths.path.join(File::PATH_SEPARATOR), "GEM_SPEC_CACHE" => Gem.paths.spec_cache_dir }.compact
 
     check_executable
 
@@ -144,7 +144,7 @@ to the same gem path as user-installed gems.
   def set_gem_exec_install_paths
     home = File.join(Gem.dir, "gem_exec")
 
-    ENV["GEM_PATH"] = ([home] + Gem.path).join(Gem.path_separator)
+    ENV["GEM_PATH"] = ([home] + Gem.path).join(File::PATH_SEPARATOR)
     ENV["GEM_HOME"] = home
     Gem.clear_paths
   end

test/rubygems/test_gem_commands_exec_command.rb

@@ -270,6 +270,7 @@ class TestGemCommandsExecCommand < Gem::TestCase
     use_ui @ui do
       util_set_arch "unknown-unknown"
       invoke "a"
+      assert_empty @ui.error
       assert_equal "Building native extensions. This could take a while...\na-2\nsometimes_used-2\nwith_platform-2\n", @ui.output
     end