Closes #87 add test for null safe toString()

Author: Alex Collins

test/unit/docker/registry/web/RegistrySpec.groovy

@@ -150,6 +150,15 @@ class RegistrySpec extends Specification {
         registry.password == null
     }
 
+    void "test toString doesn't NPE on null password :)"() {
+        when:
+        def registry = Registry.fromUrl("http://admin@172.17.42.1:5000/v1/")
+        registry.password = null
+        then:
+        "Registry{id=null, protocol='http', host='172.17.42.1', port=5000, apiVersion='v1', username='admin', password='null', repositoryService=null, version=null}"
+                .equals(registry.toString())
+    }
+
     void "test toString doesn't contain password :)"() {
         when:
         def registryStr = Registry.fromUrl("http://admin:abc123@172.17.42.1:5000/v1/").toString()