Make String#to_proc return value allow arguments.

Among other things, this allows binary operators to work:

  (:+).to_proc.call(1, 2)  # => 3

This is most commonly encountered as a result of:

    [1, 2].reduce(&:+)

(Note the ampersand.)
Merge pull request #515 from mieko/master

Make String#to_proc return value allow arguments.