Make string support even more generic

Ported from 57354de
Closes #2084

Author: d-a-v

CHANGELOG.md

@@ -3,6 +3,7 @@ ArduinoJson: change log
 
 * Add `ARDUINOJSON_STRING_LENGTH_SIZE` to the namespace name
 * Add MsgPack bin8/bin16/bin32 support (PR #2078 by @Sanae6)
+* Make string support even more generic (PR #2084 by @d-a-v)
 
 v7.0.4 (2024-03-12)
 ------

src/ArduinoJson/Strings/StringTraits.hpp

@@ -38,16 +38,15 @@ struct has_data<T,
                                            const char*>::value>::type>
     : true_type {};
 
-// size_t length() const
+// unsigned int length() const
 // - String
 
 template <class T, class = void>
 struct has_length : false_type {};
 
 template <class T>
-struct has_length<
-    T, typename enable_if<
-           is_same<decltype(declval<const T>().length()), size_t>::value>::type>
+struct has_length<T, typename enable_if<is_unsigned<
+                         decltype(declval<const T>().length())>::value>::type>
     : true_type {};
 
 // size_t size() const