[rubygems/rubygems] Support the change of did_you_mean about Exception#detailed_message

I am asking did_you_mean to use Exception#detailed_message to add
"Did you mean?" suggestion instead of overriding #message method.

https://github.com/ruby/did_you_mean/pull/177

Unfortunately, the change will affect Gem::UnknownCommandError, which
excepts did_you_mean to override #message method.

This PR absorbs the change of did_you_mean.
Gem::CommandManager now calls #detailed_message method to get a message
string with "Did you mean?" suggestion from an exception.

8f104228d3

test/rubygems/test_gem_command_manager.rb

@@ -80,7 +80,13 @@ class TestGemCommandManager < Gem::TestCase
       message << "\nDid you mean?  \"push\""
     end
 
-    assert_equal message, e.message
+    if e.respond_to?(:detailed_message)
+      actual_message = e.detailed_message(highlight: false).sub(/\A(.*?)(?: \(.+?\))/) { $1 }
+    else
+      actual_message = e.message
+    end
+
+    assert_equal message, actual_message
   end
 
   def test_run_interrupt