Why is Jersey's UriBuilder.build method encoding #'s and %'s, but not /'s?

I have a REST API which is fairly typical, except that the id's of resources are not integers, but strings, which often contain / characters. So if a customer's id is string/with/slashes then the URI for that customer should be http://localhost/customers/string%2Fwith%2Fslashes. When returning a list of customers, I want to construct that URI with a UriBuilder so I can put it in the href of ATOM-style link elements. But it doesn't quite work; here's a small test class that shows what I mean:

@Path("/customers")
public class JerseyTest {

  @Path("{id}")
  public Customer getCustomer(@PathParam("{id}") String id) {
    return null;
  }

  public static void main(String[] args) {
    buildURI("string#with#hashes"); // => http://localhost/customers/string%23with%23hashes
    buildURI("string/with/slashes"); // => http://localhost/customers/string/with/slashes
  }

  public static void buildURI(String id) {
    UriBuilder builder = UriBuilder.fromUri("http://localhost");
    builder.path(JerseyTest.class).path(JerseyTest.class, "getCustomer");
    URI uri = builder.build(id);
    System.out.println(uri);
  }
}

The #'s get encoded as I would expect but the /'s don't. I tried using builder.build(URLEncoder.encode(id)) instead, but then the UriBuilder encodes the %'s so you get .../string%252Fwith%252Fslashes!

It seems inconsistent to me that it encodes # and % but not /, but I suspect there is a good reason for it which I am not seeing. So my question is:

1. How can I get UriBuilder to give me .../string%2Fwith%2Fslashes, which is the URI that causes Jersey to call getCustomer with id equal to st