java.net.URL should be (de)serializable out of the box

[magro]

roundTrip(57, 57, new URL("https://github.com/EsotericSoftware/kryo")); fails with

java.lang.NullPointerException
        at java.net.URLStreamHandler.sameFile(URLStreamHandler.java:407)
        at java.net.URLStreamHandler.equals(URLStreamHandler.java:333)
        at java.net.URL.equals(URL.java:870)
        at org.junit.Assert.isEquals(Assert.java:131)
        at org.junit.Assert.equalsRegardingNull(Assert.java:127)
        at org.junit.Assert.assertEquals(Assert.java:111)
        at org.junit.Assert.assertEquals(Assert.java:144)
        at com.esotericsoftware.kryo.KryoTestCase.doAssertEquals(KryoTestCase.java:269)
        at com.esotericsoftware.kryo.KryoTestCase.roundTripWithStreamFactory(KryoTestCase.java:209)
        at com.esotericsoftware.kryo.KryoTestCase.roundTrip(KryoTestCase.java:81)

This was originally reported in magro/memcached-session-manager#283