fix: filter false values from empty References headers

When an email has an empty References header alongside a populated one,
ensureMessageIDFormat returns false for the empty value, resulting in
[false, '<id@example.com>'] instead of the expected string output.

Fixes #385

Author: andris9

lib/mail-parser.js

@@ -348,7 +348,7 @@ class MailParser extends Transform {
                     } catch (E) {
                         // ignore
                     }
-                    value = value.split(/\s+/).map(this.ensureMessageIDFormat);
+                    value = value.split(/\s+/).map(this.ensureMessageIDFormat).filter(val => val);
                     break;
                 case 'message-id':
                 case 'in-reply-to':

test/issue-385-test.js

@@ -0,0 +1,25 @@
+'use strict';
+
+const { simpleParser } = require('..');
+
+module.exports['Issue 385'] = {
+    'should not include false for empty References header': test => {
+        let raw = [
+            'From: sender@example.com',
+            'To: recipient@example.com',
+            'Subject: Test',
+            'References:',
+            'References: <some.message.id@example.com>',
+            'MIME-Version: 1.0',
+            'Content-Type: text/plain',
+            '',
+            'Test body'
+        ].join('\r\n');
+
+        simpleParser(raw, (err, mail) => {
+            test.ifError(err);
+            test.equal(mail.references, '<some.message.id@example.com>');
+            test.done();
+        });
+    }
+};