Explicitly disable copy assignment operator

include/rapidjson/document.h

@@ -326,6 +326,8 @@ private:
     //! Disallow construction from non-const array
     template<SizeType N>
     GenericStringRef(CharType (&str)[N]) /* = delete */;
+    //! Copy assignment operator not permitted - immutable type
+    GenericStringRef& operator=(const GenericStringRef& rhs) /* = delete */;
 };
 
 //! Mark a character pointer as constant string