added support for default headers in Browser PHP and moved default header user-agent to the default headers.

Author: 51imyyy

README.md

@@ -2388,7 +2388,6 @@ given setting applied.
 The `withHeader(string $header, string $value): Browser` method can be used to
 add a request header for all following requests.
 ```php
-$browser->get($url)->then(function (Psr\Http\Message\ResponseInterface $response) {
     var_dump($response);
 }, function (Exception $e) {
     echo 'Error: ' . $e->getMessage() . PHP_EOL;
@@ -2406,7 +2405,6 @@ remove any default request headers previously set via
 the [`withHeader()` method](#withheader).
 
 ```php
-$browser->get($url)->then(function (Psr\Http\Message\ResponseInterface $response) {
     var_dump($response);
 }, function (Exception $e) {
     echo 'Error: ' . $e->getMessage() . PHP_EOL;