fix(mailgun): preserve duplicate custom headers

[faisalahammad]

Summary

Preserves multiple headers with the same key when sending through the Mailgun transport. RFC 5322 section 3.6 permits repeated optional-field headers and Mailgun documents h:X-Mailgun-Tag as accepting an array of strings, but PostmanMailgunMailEngine was flat-assigning $mailgunMessage["h:" . $name] on every iteration so only the last value survived.

Fixes #74

Changes

Postman/Postman-Mail/PostmanMailgunMailEngine.php

Before:

private function addHeader( $name, $value, $deprecated = "" ) {
    if ( $value && ! empty( $value ) ) {
        $this->mailgunMessage["h:" . $name] = preg_replace("/.*:\s?/", "", $value);
    }
}

After:

private function addHeader( $name, $value, $append = false ) {
    if ( $value && ! empty( $value ) ) {
        $clean = preg_replace( "/.*:\s?/", "", $value );
        $key   = "h:" . $name;

        if ( $append && isset( $this->mailgunMessage[ $key ] ) ) {
            $existing = $this->mailgunMessage[ $key ];
            if ( ! is_array( $existing ) ) {
                $existing = array( $existing );
            }
            $this->mailgunMessage[ $key ] = array_merge( $existing, array( $clean ) );
        } else {
            $this->mailgunMessage[ $key ] = $clean;
        }
    }
}

Why: the third arg was a dead $deprecated placeholder. Rename it to $append=false, and when the header loop at line 197 sets it to true and the same key is already present, append instead of overwrite. The first occurrence still hits the else branch, so single-value behaviour is byte-identical.

Postman/Postman-Mail/Services/MailGun/Handler.php

Before: the multipart branch only activated when attachment was set.
After: also activates when any content value is an array.
Why: when headers are accumulated as arrays, wp_remote_post body serialisation would have produced h:X-Mailgun-Tag[0] and [1] query keys, which Mailgun would have parsed as literal header names instead of repeated tags. Routing duplicate header tags through the existing multipart emitter keeps the wire shape Mailgun expects.

Repro

add_action("init", function () {
    if ( ! isset( $_GET["mailgun_tags_test"] ) ) return;
    wp_mail(
        "you@example.com",
        "Multi-tag test",
        "Body",
        array(
            "X-Mailgun-Tag: test-category-account",
            "X-Mailgun-Tag: test-account-reset-password",
        )
    );
});

Before: Mailgun dashboard log shows one tag. After: both tags appear.

Testing

Test 1: duplicate tags delivered

1. Configure Post SMTP for the Mailgun transport.
2. Trigger the snippet above.
3. Open the Mailgun dashboard log for the message.
   Result: both test-category-account and test-account-reset-password are listed as tags. ✓

Test 2: single tag still works

1. Replace both lines with one X-Mailgun-Tag: single-tag.
2. Trigger the snippet.
   Result: one tag. ✓ (No regression for single-value behaviour.)

Test 3: attachment still works

1. Send a normal Mailgun email with a file attachment and no custom headers.
2. Confirm the message delivers and the attachment arrives intact.
   Result: byte-identical to before. ✓ (No regression in the multipart path.)

Notes

• Other API engines (SendGrid, SMTP2Go, MailerSend, Mailjet, Sendinblue, Sendpulse, Postmark, SparkPost) have the same flat-assign pattern. They are out of scope for this PR by maintainer decision. A follow-up could tackle them with the same one-liner.
• No composer / phpunit / phpcs in this repo, so verification was an off-tree reproduction harness plus the manual scenarios above.