Roman Numeral Kata

While working on the Roman Numeral Kata, I was bothered by the existence of special pairs in my numerals hash.
class Converter
    NUMERALS = { 50 => "L", 40 => "XL", 10 => "X", 9 => "IX", 5 => "V", 4 => "IV", 1 => "I" }
    def convert(number)
        return NUMERALS[number] if NUMERALS[number]
        to_roman(number) 
    end

    private

    def to_roman(number)
        result = ""
        NUMERALS.each do |value, numeral|
            while number >= value
                result += numeral
                number -= value
            end 
        end
        result
    end
end

Notice the 40, 9, and 4. These aren't unique numerals, rather they are pairs that represent special subtraction rules. Doing this gets my tests to pass and solves the problem, but I wanted to see if I could come up with a way that better represented how roman numerals work.

Looking for inspiration, I asked twitter and google for some help, but everything I found either used this technique or did some magic number substitution (YUCK). So I set out to find my own solution. Notice I am not professing I set out to find a better solution, only my own, alternative solution.

I won't be showing all of the tests as we go along, but the entire solution was test driven with RSpec. You can review the history and see the tests on github.