Make 'RUPNP::ControlPoint #search_only should detect devices' spec pass.

* lib/rupnp/cp/remote_device.rb:
  #extract_icons : add a return if icons_list is empty.

* spec/spec_helper.rb:
  add webmoxk require
  add #generate_xml_device_description helper method to generate a
  Device description.

* tasks/gem.rake: add webmock

* spec/control_point_spec.rb: pass!