Conditional Conversion support

    public static class SkipEmptyCollectionConverter extends CollectionConverter implements ConditionalConverter {
        public SkipEmptyCollectionConverter(Mapper mapper) {
            super(mapper);
        }

        public boolean shouldConvert(Class type, Object value) {
            return (value != null && ((Collection) value).size() != 0);
        }
    }

    public static class SkipEmptyMapConverter extends MapConverter implements ConditionalConverter {
        public SkipEmptyMapConverter(Mapper mapper) {
            super(mapper);
        }

        public boolean shouldConvert(Class type, Object value) {
            return (value != null && ((Map) value).size() != 0);
        }
    }

    public static class SkipEmptyStringConverter extends StringConverter {
        public boolean shouldConvert(Class type, Object value) {
            return (value != null && ((String)value).length() != 0);
        }
    }

    public static class Contact extends StandardObject {
        public String name;
        public String company;
        public List emailAddresses = new ArrayList();
        public Map addresses = new HashMap();
        public Map telephone = new HashMap();
        public String notes;

        public Contact(String name) {
            this.name = name;
        }
    }

    public void testSkipNonSimpleTypes() {
        List addressBook = new ArrayList();
        Contact wallace = new Contact("Wallace");
        wallace.company = "Window Washing Inventors, Inc.";
        wallace.addresses.put("home", "62 West Wallaby Street");
        addressBook.add(wallace);

        Contact gromit = new Contact("Gromit");
        gromit.emailAddresses.add("gromit@genious-k9s.net");
        addressBook.add(gromit);

        Contact jenny = new Contact("Jenny");
        jenny.telephone.put("home", "867-5309");
        jenny.notes = "Found this number on the bathroom wall.";
        addressBook.add(jenny);

        xstream.alias("contact", Contact.class);
        xstream.registerConverter(new SkipEmptyCollectionConverter(xstream.getMapper()));
        xstream.registerConverter(new SkipEmptyMapConverter(xstream.getMapper()));
        xstream.registerConverter(new SkipEmptyStringConverter());

        String expectedXml = ""+
                "<list>\n" +
                "  <contact>\n" +
                "    <name>Wallace</name>\n" +
                "    <company>Window Washing Inventors, Inc.</company>\n" +
                "    <addresses>\n" +
                "      <entry>\n" +
                "        <string>home</string>\n" +
                "        <string>62 West Wallaby Street</string>\n" +
                "      </entry>\n" +
                "    </addresses>\n" +
                "  </contact>\n" +
                "  <contact>\n" +
                "    <name>Gromit</name>\n" +
                "    <emailAddresses>\n" +
                "      <string>gromit@genious-k9s.net</string>\n" +
                "    </emailAddresses>\n" +
                "  </contact>\n" +
                "  <contact>\n" +
                "    <name>Jenny</name>\n" +
                "    <telephone>\n" +
                "      <entry>\n" +
                "        <string>home</string>\n" +
                "        <string>867-5309</string>\n" +
                "      </entry>\n" +
                "    </telephone>\n" +
                "    <notes>Found this number on the bathroom wall.</notes>\n" +
                "  </contact>\n" +
                "</list>";

        assertBothWays(addressBook, expectedXml);
    }