Mail: reset Encoding to 8-bit in wp_mail().

The PHPMailer library may automatically switch its encoding based on various internal factors.

This commit fixes a bug where the `$phpmailer` global was unintentionally persisting its `Encoding` property from the first `wp_mail()` call to all subsequent calls.

This includes unit tests to verify the fix is accurate, and a change to the mock-mailer helper that worked around this bug just-in-time when running the test suite.

Props codebuddy, dilip2615, rishabhwp, sajjad67, sirlouen, stephenharris.

Fixes #33972.
Built from https://develop.svn.wordpress.org/trunk@61131


git-svn-id: http://core.svn.wordpress.org/trunk@60467 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/pluggable.php

@@ -384,6 +384,15 @@ if ( ! function_exists( 'wp_mail' ) ) :
 		$phpmailer->Body    = '';
 		$phpmailer->AltBody = '';
 
+		/*
+		 * Reset encoding to 8-bit, as it may have been automatically downgraded
+		 * to 7-bit by PHPMailer (based on the body contents) in a previous call
+		 * to wp_mail().
+		 *
+		 * See https://core.trac.wordpress.org/ticket/33972
+		 */
+		$phpmailer->Encoding = PHPMailer\PHPMailer\PHPMailer::ENCODING_8BIT;
+
 		// Set "From" name and email.
 
 		// If we don't have a name from the input headers.