refactor: clear PHPMailer encoding to prevent stale state between wp_mail() calls

Author: R1shabh-Gupta

src/wp-includes/pluggable.php

@@ -358,7 +358,7 @@ function wp_mail( $to, $subject, $message, $headers = '', $attachments = array()
 		$phpmailer->clearReplyTos();
 		$phpmailer->Body     = '';
 		$phpmailer->AltBody  = '';
-		$phpmailer->Encoding = PHPMailer\PHPMailer\PHPMailer::ENCODING_8BIT;
+		$phpmailer->Encoding = '';
 
 		// Set "From" name and email.
 

tests/phpunit/includes/mock-mailer.php

@@ -20,7 +20,6 @@ class MockPHPMailer extends WP_PHPMailer {
 	public $mock_sent = array();
 
 	public function preSend() {
-		$this->Encoding = '8bit';
 		return parent::preSend();
 	}
 

tests/phpunit/tests/pluggable/wpMail.php

@@ -567,9 +567,9 @@ public function test_wp_mail_encoding_does_not_bleed() {
 		$mailer = tests_retrieve_phpmailer_instance();
 		$this->assertEquals( 'quoted-printable', $mailer->Encoding );
 
-		wp_mail( WP_TESTS_EMAIL, 'A follow up short email', 'Short email –' );
+		wp_mail( WP_TESTS_EMAIL, 'A follow up short email', 'Short email' );
 
 		$mailer = tests_retrieve_phpmailer_instance();
-		$this->assertEquals( '8bit', $mailer->Encoding );
+		$this->assertEquals( '', $mailer->Encoding );
 	}
 }