Document return values for non-existant headers

See php-fig/fig-standards#450

Author: evert

src/MessageInterface.php

@@ -86,14 +86,20 @@ public function hasHeader($name);
      * NOTE: Not all header values may be appropriately represented using
      * comma concatenation. For such headers, use getHeaderLines() instead
      * and supply your own delimiter when concatenating.
+     * 
+     * If the header did not appear in the message, this method should return
+     * a null value.
      *
      * @param string $name Case-insensitive header field name.
-     * @return string
+     * @return string|null
      */
     public function getHeader($name);
 
     /**
      * Retrieves a header by the given case-insensitive name as an array of strings.
+     * 
+     * If the header did not appear in the message, this method should return an
+     * empty array.
      *
      * @param string $name Case-insensitive header field name.
      * @return string[]