Add convenience method Key(std::basic_string<Ch> const&) to Writer

include/rapidjson/writer.h

@@ -222,6 +222,13 @@ public:
 
     bool Key(const Ch* str, SizeType length, bool copy = false) { return String(str, length, copy); }
 
+#if RAPIDJSON_HAS_STDSTRING
+    bool Key(const std::basic_string<Ch>& str)
+    {
+      return Key(str.data(), SizeType(str.size()));
+    }
+#endif
+	
     bool EndObject(SizeType memberCount = 0) {
         (void)memberCount;
         RAPIDJSON_ASSERT(level_stack_.GetSize() >= sizeof(Level)); // not inside an Object