Add expected method MailBuilder->withHeaders

- Address https://github.com/friendica/friendica/issues/9250#issuecomment-696055412

src/Util/EMailer/MailBuilder.php

@@ -159,6 +159,36 @@ abstract class MailBuilder
 		return $this;
 	}
 
+	/**
+	 * Returns the current headers
+	 *
+	 * @return string[][]
+	 */
+	public function getHeaders()
+	{
+		return $this->headers;
+	}
+
+	/**
+	 * Sets the headers
+	 *
+	 * Expected format is
+	 * [
+	 *   'Header1' => ['value1', 'value2', ...],
+	 *   'Header2' => ['value3', 'value4', ...],
+	 *   ...
+	 * ]
+	 *
+	 * @param string[][] $headers
+	 * @return $this
+	 */
+	public function withHeaders(array $headers)
+	{
+		$this->headers = $headers;
+
+		return $this;
+	}
+
 	/**
 	 * Adds a value to a header
 	 *