Added string as return types for __toString

Author: Aleksandr Golouz

src/StreamInterface.php

@@ -25,7 +25,7 @@ interface StreamInterface
      * @see http://php.net/manual/en/language.oop5.magic.php#object.tostring
      * @return string
      */
-    public function __toString();
+    public function __toString(): string;
 
     /**
      * Closes the stream and any underlying resources.

src/UriInterface.php

@@ -320,5 +320,5 @@ public function withFragment(string $fragment): UriInterface;
      * @see http://tools.ietf.org/html/rfc3986#section-4.1
      * @return string
      */
-    public function __toString();
+    public function __toString(): string;
 }