add tests for Parser

src/main/java/com/github/nkzawa/engineio/parser/Packet.java

@@ -15,14 +15,12 @@ public class Packet {
     public String type;
     public String data;
 
+    public Packet(String type) {
+        this(type, null);
+    }
+
     public Packet(String type, String data) {
         this.type = type;
         this.data = data;
     }
-
-    @Override
-    public String toString() {
-        return String.format("{\"type\": \"%s\", \"data\": \"%s\"}", this.type, this.data);
-    }
-
 }