Use (string) $body instead of $body->getContents() for getting contents from StreamInterface. (#2871)

because method `getContents()` only returns the remaining contents in a string.

Author: limingxinleo

src/Base/Response.php

@@ -102,7 +102,7 @@ class Response implements ResponseInterface
 
     public function __toString()
     {
-        return $this->getBody()->getContents();
+        return (string) $this->getBody();
     }
 
     /**

src/Server/Request.php

@@ -480,7 +480,7 @@ protected static function normalizeParsedBody(array $data = [], ?RequestInterfac
 
         try {
             $parser = static::getParser();
-            if ($parser->has($contentType) && $content = $request->getBody()->getContents()) {
+            if ($parser->has($contentType) && $content = (string) $request->getBody()) {
                 $data = $parser->parse($content, $contentType);
             }
         } catch (\InvalidArgumentException $exception) {