MySqlPlatform: Fix default isolation level

Default isolation level on mysql is REPEATABLE READ

https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_tx_isolation

lib/Doctrine/DBAL/Platforms/MySqlPlatform.php

@@ -25,6 +25,7 @@ use Doctrine\DBAL\Schema\Index;
 use Doctrine\DBAL\Schema\Table;
 use Doctrine\DBAL\Types\BlobType;
 use Doctrine\DBAL\Types\TextType;
+use Doctrine\DBAL\Connection;
 
 /**
  * The MySqlPlatform provides the behavior, features and SQL dialect of the
@@ -1126,4 +1127,12 @@ class MySqlPlatform extends AbstractPlatform
 
         return parent::quoteStringLiteral($str);
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function getDefaultTransactionIsolationLevel()
+    {
+        return Connection::TRANSACTION_REPEATABLE_READ;
+    }
 }

tests/Doctrine/Tests/DBAL/Platforms/MySqlPlatformTest.php

@@ -3,6 +3,7 @@
 namespace Doctrine\Tests\DBAL\Platforms;
 
 use Doctrine\DBAL\Platforms\MySqlPlatform;
+use Doctrine\DBAL\Connection;
 
 class MySqlPlatformTest extends AbstractMySQLPlatformTestCase
 {
@@ -10,4 +11,12 @@ class MySqlPlatformTest extends AbstractMySQLPlatformTestCase
     {
         return new MysqlPlatform;
     }
+
+    public function testHasCorrectDefaultTransactionIsolationLevel()
+    {
+        $this->assertEquals(
+            Connection::TRANSACTION_REPEATABLE_READ,
+            $this->_platform->getDefaultTransactionIsolationLevel()
+        );
+    }
 }