Don't pollute toplevel by an IRB internal method

There actually are some libraries that requires 'irb' such as byebug, and when
'irb' is required, it requires irb/src_encoding.rb, then it defines the toplevel
default_src_encoding method that is visible from anywhere in the end users' apps.

Here's a quick oneliner that shows what's happening.

  % ruby -rpry-byebug -e 'p private_methods(false)'
  [:include, :using, :define_method, :public, :private, :DelegateClass, :default_src_encoding]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59090 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/irb/magic-file.rb

@@ -10,7 +10,7 @@ module IRB
       line = io.gets if line[0,2] == "#!"
       encoding = detect_encoding(line)
       internal_encoding = encoding
-      encoding ||= default_src_encoding
+      encoding ||= IRB.default_src_encoding
       io.rewind
       io.set_encoding(encoding, internal_encoding)
 

lib/irb/src_encoding.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: false
 # DO NOT WRITE ANY MAGIC COMMENT HERE.
-def default_src_encoding
-  return __ENCODING__
+module IRB
+  def self.default_src_encoding
+    return __ENCODING__
+  end
 end