ERB#result does not accept a proc.

The source shows that #result delegates to Kernel.eval.  The
documentation for Kernel.eval says:

  Evaluates the Ruby expression(s) in string. If binding is given, which
  must be a Binding object...

Furthermore, I demonstrated that #result does not accept a proc by
switching the example in the documentation to use a proc:

```ruby
require "erb"

class Listings
  PRODUCT = { :name => "Chicken Fried Steak",
              :desc => "A well messages pattie, breaded and fried.",
              :cost => 9.95 }

  attr_reader :product, :price

  def initialize( product = "", price = "" )
    @product = product
    @price = price
  end

  def build
    # b = binding
    b = proc {}
    # create and run templates, filling member data variables
    ERB.new(<<-'END_PRODUCT'.gsub(/^\s+/, ""), 0, "", "@product").result b
      <%= PRODUCT[:name] %>
      <%= PRODUCT[:desc] %>
    END_PRODUCT
    ERB.new(<<-'END_PRICE'.gsub(/^\s+/, ""), 0, "", "@price").result b
      <%= PRODUCT[:name] %> -- <%= PRODUCT[:cost] %>
      <%= PRODUCT[:desc] %>
    END_PRICE
  end
end

listings = Listings.new
listings.build

puts listings.product + "\n" + listings.price
```

This outputs:

```
ruby-2.1.1/lib/ruby/2.1.0/erb.rb:847:in `eval': wrong argument type proc (expected binding) (TypeError)
	from ruby-2.1.1/lib/ruby/2.1.0/erb.rb:847:in `block in result'
	from ruby-2.1.1/lib/ruby/2.1.0/erb.rb:848:in `call'
	from ruby-2.1.1/lib/ruby/2.1.0/erb.rb:848:in `result'
	from -:21:in `build'
	from -:34:in `block in <main>'
	from -:37:in `call'
	from -:37:in `<main>'
```