validates_length_of should exclude last member of an end-excluding range

If provided with an end-excluding range, validates_length_of will not exclude the last member of the range.

In the code below, the current behavior of validates_length_of will consider the first_name attribute valid if it is 61 characters long.

class Person << ActiveRecord::Base
  validates_length_of :first_name, :in => 3...61
end

The attached patch corrects this by properly excluding the last member of an exclusive range.