Copy config to make IRB::Context#use_colorize? functional

on initialize

This fixes https://github.com/ruby/ruby/pull/2188

lib/irb/context.rb

@@ -40,6 +40,7 @@ module IRB
       @load_modules = IRB.conf[:LOAD_MODULES]
 
       @use_readline = IRB.conf[:USE_READLINE]
+      @use_colorize = IRB.conf[:USE_COLORIZE]
       @verbose = IRB.conf[:VERBOSE]
       @io = nil
 

test/irb/test_context.rb

@@ -75,5 +75,9 @@ module TestIRB
     ensure
       $VERBOSE = verbose
     end
+
+    def test_default_config
+      assert_equal(true, @context.use_colorize?)
+    end
   end
 end