Calculates the MD5 digest and returns the value

The method name was not really explicit. Indeed, we
expect the method to return the md5 digest but it
returns something else.

lib/src/string.dart

@@ -115,9 +115,12 @@ extension StringX on String {
 
   List<int> toUtf16() => codeUnits;
 
-  String get md5 {
-    var utf8 = toUtf8();
-    var digest = crypto.md5.convert(utf8);
-    return String.fromCharCodes(digest.bytes);
-  }
+  /// Calculates the MD5 digest and returns the value as a [String] of
+  /// hexadecimal digits.
+  ///
+  /// ```dart
+  /// print('abc'.md5); //900150983cd24fb0d6963f7d28e17f72
+  /// print('message digest'.md5); //f96b697d7cb7938d525a2f31aaf161d0
+  /// ```
+  String get md5 => crypto.md5.convert(toUtf8()).toString();
 }

test/string_test.dart

@@ -1,7 +1,7 @@
 import 'dart:convert';
 
-import 'package:test/test.dart';
 import 'package:dartx/dartx.dart';
+import 'package:test/test.dart';
 
 void main() {
   group('StringX', () {
@@ -121,5 +121,26 @@ void main() {
       expect('hello'.toUtf16(), 'hello'.codeUnits);
       expect('ഐ⌛酪Б👨‍👨‍👧‍👦'.toUtf16(), 'ഐ⌛酪Б👨‍👨‍👧‍👦'.codeUnits);
     });
+
+    test('.md5)', () {
+      expect(''.md5, 'd41d8cd98f00b204e9800998ecf8427e');
+      expect('a'.md5, '0cc175b9c0f1b6a831c399e269772661');
+      expect('abc'.md5, '900150983cd24fb0d6963f7d28e17f72');
+      expect('message digest'.md5, 'f96b697d7cb7938d525a2f31aaf161d0');
+      expect('ഐ⌛酪Б👨‍👨‍👧‍👦'.md5, 'c7834eff7c967101cfb65b8f6d15ad46');
+      expect(
+          'abcdefghijklmnopqrstuvwxyz'.md5,
+          'c3fcd3d76192e4007dfb496cca67e13b'
+      );
+      expect(
+          'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789'.md5,
+          'd174ab98d277d9f5a5611c2c9f419d9f'
+      );
+      expect(
+          '12345678901234567890123456789012345678901234567890123456789012'
+              '345678901234567890'.md5,
+          '57edf4a22be3c955ac49da2e2107b67a'
+      );
+    });
   });
 }