Try to fix a clang missing assignment warning

example/serialize/serialize.cpp

@@ -14,6 +14,12 @@ public:
     Person(const Person& rhs) : name_(rhs.name_), age_(rhs.age_) {}
     virtual ~Person();
 
+    Person& operator=(const Person& rhs) {
+        name_ = rhs.name_;
+        age_ = rhs.age_;
+        return *this;
+    }
+
 protected:
     template <typename Writer>
     void Serialize(Writer& writer) const {
@@ -107,6 +113,13 @@ public:
     Employee(const Employee& rhs) : Person(rhs), dependents_(rhs.dependents_), married_(rhs.married_) {}
     virtual ~Employee();
 
+    Employee& operator=(const Employee& rhs) {
+        static_cast<Person&>(*this) = rhs;
+        dependents_ = rhs.dependents_;
+        married_ = rhs.married_;
+        return *this;
+    }
+
     void AddDependent(const Dependent& dependent) {
         dependents_.push_back(dependent);
     }