Let irb use an empty file as irbrc

to get rid of side-effect by existing .irbrc file.

Author: nobu

core/binding/fixtures/irbrc

@@ -0,0 +1 @@
+# empty configuration

core/binding/irb_spec.rb

@@ -4,8 +4,9 @@
   describe "Binding#irb" do
     it "creates an IRB session with the binding in scope" do
       irb_fixture = fixture __FILE__, "irb.rb"
+      irbrc_fixture = fixture __FILE__, "irbrc"
 
-      out = IO.popen([*ruby_exe, irb_fixture], "r+") do |pipe|
+      out = IO.popen([{"IRBRC"=>irbrc_fixture}, *ruby_exe, irb_fixture], "r+") do |pipe|
         pipe.puts "a ** 2"
         pipe.puts "exit"
         pipe.readlines.map(&:chomp)