Add toString() implementation.

Author: Quintin Willison

android/src/main/java/io/ably/lib/types/RegistrationToken.java

@@ -33,4 +33,12 @@ public String toName() {
             return name().toLowerCase();
         }
     }
+
+    @Override
+    public String toString() {
+        return "RegistrationToken{" +
+            "type=" + type +
+            ", token='" + token + '\'' +
+            '}';
+    }
 }