fixed typehints (#61, #63)

Author: janbarasek

src/Mail/Message.php

@@ -66,7 +66,7 @@ public function setFrom(string $email, string $name = null)
 	/**
 	 * Returns the sender of the message.
 	 */
-	public function getFrom(): array
+	public function getFrom(): ?array
 	{
 		return $this->getHeader('From');
 	}
@@ -163,7 +163,7 @@ public function setReturnPath(string $email)
 	/**
 	 * Returns the Return-Path header.
 	 */
-	public function getReturnPath(): string
+	public function getReturnPath(): ?string
 	{
 		return $this->getHeader('Return-Path');
 	}
@@ -183,7 +183,7 @@ public function setPriority(int $priority)
 	/**
 	 * Returns email priority.
 	 */
-	public function getPriority(): int
+	public function getPriority(): ?int
 	{
 		return $this->getHeader('X-Priority');
 	}