Flattening hash into string in Ruby

Is there a way to flatten a hash into a string, with optional delimiters between keys and values, and key/value pairs?

For example, print {:a => :b, :c => :d}.flatten('=','&') should print a=b&c=d

I wrote some code to do this, but I was wondering if there was a neater way:

class Hash
  def flatten(keyvaldelimiter, entrydelimiter)
    string = ""
    self.each do
      |key, value|
      key = "#{entrydelimiter}#{key}" if string != "" #nasty hack
      string += "#{key}#{keyvaldelimiter}#{value}"  
    end
    return string
  end
end

print {:a => :b, :c => :d}.flatten('=','&') #=> 'c=d&a=b'

Thanks