PT-2377 - fixed pt-table-sync for JSON utf8 strings

The MySQL driver DBD::mysql does not decode JSON values as utf8
although MySQL uses utf8mb4 for all JSON strings.

This change decodes JSON values as utf8 (when not already done)
such that SQL statements are generated correctly.

t/pt-table-sync/samples/pt-2377.sql

@@ -0,0 +1,14 @@
+DROP DATABASE IF EXISTS test;
+CREATE DATABASE test;
+USE test;
+
+CREATE TABLE `test_table` (
+  `id` INT AUTO_INCREMENT PRIMARY KEY,
+  `data` JSON NOT NULL
+) ENGINE=InnoDB;
+
+INSERT INTO
+  `test_table` (`data`)
+VALUES
+  ('{"name": "Müller"}'),
+  ('{"reaction": "哈哈"}');