Make implicit nullable types explicit (#359)

src/Body/FormBody.php

@@ -30,7 +30,7 @@ final class FormBody implements RequestBody
     /**
      * @param string $boundary An optional multipart boundary string
      */
-    public function __construct(string $boundary = null)
+    public function __construct(?string $boundary = null)
     {
         /** @noinspection PhpUnhandledExceptionInspection */
         $this->boundary = $boundary ?? \bin2hex(\random_bytes(16));

src/Connection/Internal/Http1Parser.php

@@ -110,7 +110,7 @@ public function buffer(string $data): void
      *
      * @throws ParseException
      */
-    public function parse(string $data = null): ?Response
+    public function parse(?string $data = null): ?Response
     {
         if ($data !== null) {
             $this->buffer .= $data;

src/InvalidRequestException.php

@@ -7,7 +7,7 @@ final class InvalidRequestException extends HttpException
     /** @var Request */
     private $request;
 
-    public function __construct(Request $request, string $message, int $code = 0, \Throwable $previous = null)
+    public function __construct(Request $request, string $message, int $code = 0, ?\Throwable $previous = null)
     {
         parent::__construct($message, $code, $previous);
         $this->request = $request;

src/ParseException.php

@@ -9,7 +9,7 @@ final class ParseException extends HttpException
      * @param int             $code
      * @param \Throwable|null $previousException
      */
-    public function __construct(string $message, int $code, \Throwable $previousException = null)
+    public function __construct(string $message, int $code, ?\Throwable $previousException = null)
     {
         parent::__construct($message, $code, $previousException);
     }