jaxrs: add support for plugin specific properties

They should be double encoded, i.e.

  { "city" => "Carrot City", "billing_address" => "address[postcode]=12345" }

should be sent as:

pluginProperty=city%3DCarrot%20City&pluginProperty=billing_address%3Daddress%5Bpostcode%5D%3D12345

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>