make return type of request->getUploadedFile nullable (#3)

Author: nashgao

composer.json

@@ -19,7 +19,7 @@
     "issues": "https://github.com/swlib/http/issues"
   },
   "require": {
-    "php": ">=7.0",
+    "php": ">=7.1",
     "psr/http-message": "~1.0"
   },
   "autoload": {

src/Request.php

@@ -216,7 +216,7 @@ public function withParsedBody($data): self
         return $this;
     }
 
-    public function getUploadedFile(string $name): UploadedFileInterface
+    public function getUploadedFile(string $name): ?UploadedFileInterface
     {
         return $this->uploadedFiles[$name] ?? null;
     }