Add BodyWritable / BodyReadable for form data

[wsargent]

The Java API does not have a body writable for java.util.Map[String, Array[String]] -- so you can't write the theoretically clear:

Map<String, List<String>> formData = parseFormData("key1=value1&key2=value2");
CompletionStage<WSResponse> responseStage = ws.url().post(formData);

Likewise, you'd like to be able to read data back out:

Map<String, List<String>> formData = response.body(formData());