Add std::string overload to PrettyWriter when RAPIDJSON_HAS_STDSTRING defined.

include/rapidjson/prettywriter.h

@@ -82,6 +82,12 @@ public:
         return Base::WriteString(str, length);
     }
 
+#if RAPIDJSON_HAS_STDSTRING
+    bool String(const std::basic_string<Ch>& str) {
+      return String(str.data(), SizeType(str.size()));
+    }
+#endif
+
     bool StartObject() {
         PrettyPrefix(kObjectType);
         new (Base::level_stack_.template Push<typename Base::Level>()) typename Base::Level(false);