Ruby, How to add a param to an URL that you don't know if it has any other param already

require 'uri'

uri = URI("http://url.com?p1=v1&p2=2")
ar = URI.decode_www_form(uri.query) << ["param","value"]
uri.query = URI.encode_www_form(ar)
p uri #=> #<URI::HTTP:0xa0c44c8 URL:http://url.com?p1=v1&p2=2&param=value>

uri = URI("http://url.com")
uri.query = "param=value" if uri.query.nil?
p uri #=> #<URI::HTTP:0xa0eaee8 URL:http://url.com?param=value>

EDIT:(by fguillen, to merge all the good propositions and also to make it compatible with his question test suite.)

require 'uri'

def add_param(url, param_name, param_value)
  uri = URI(url)
  params = URI.decode_www_form(uri.query || "") << [param_name, param_value]
  uri.query = URI.encode_www_form(params)
  uri.to_s
end

More elegant solution:

url       = 'http://example.com?exiting=0'
params    = {new_param: 1}
uri       = URI.parse url
uri.query = URI.encode_www_form URI.decode_www_form(uri.query || '').concat(params.to_a)
uri.to_s #=> http://example.com?exiting=0&new_param=1