When given Symbol names, {Single}Forwardable#def_delegators should reject #__id__ and #__send__ as String names.

I couldn't find `test_forwardable.rb`.

`ruby -v` ruby 2.1.0dev (2013-08-27 trunk 42710) [x86_64-linux]

```ruby
# coding: us-ascii

$VERBOSE = true

require 'forwardable'

class Foo
  extend Forwardable

  attr_reader :destination

  def initialize
    @destination = []
  end

  def_delegators :@destination, '__id__'
end

foo = Foo.new

p foo.destination.__id__.equal?(foo.__id__) #=> false

class Foo
  def_delegators :@destination, :__id__
end

p foo.destination.__id__.equal?(foo.__id__) #=> true


DESTINATION = []
obj = Object.new
obj.extend SingleForwardable

obj.def_delegators :DESTINATION, '__id__'
p obj.__id__.equal?(DESTINATION.__id__) #=> false

obj.def_delegators :DESTINATION, :__id__
p obj.__id__.equal?(DESTINATION.__id__) #=> true
```