add `Key()` method to `Writer` and `PrettyWriter`

include/rapidjson/prettywriter.h

@@ -88,6 +88,8 @@ public:
         return Base::WriteStartObject();
     }
 
+    bool Key(const Ch* str, SizeType length, bool copy = false) { return String(str, length, copy); }
+	
     bool EndObject(SizeType memberCount = 0) {
         (void)memberCount;
         RAPIDJSON_ASSERT(Base::level_stack_.GetSize() >= sizeof(typename Base::Level));

include/rapidjson/writer.h

@@ -131,6 +131,8 @@ public:
         return WriteStartObject();
     }
 
+    bool Key(const Ch* str, SizeType length, bool copy = false) { return String(str, length, copy); }
+	
     bool EndObject(SizeType memberCount = 0) {
         (void)memberCount;
         RAPIDJSON_ASSERT(level_stack_.GetSize() >= sizeof(Level));