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

Author: 51imyyy

tests/BrowserTest.php

@@ -504,19 +504,6 @@ public function testCancelGetRequestShouldCancelUnderlyingSocketConnection()
         $promise->cancel();
     }
 
-    public function testWithElseHeader()
-    {
-        $this->browser = $this->browser->withHeader('User-Agent', 'ACMC');
-
-        $that = $this;
-        $this->sender->expects($this->once())->method('send')->with($this->callback(function (RequestInterface $request) use ($that) {
-            $that->assertEquals(array('ACMC'), $request->getHeader('User-Agent'));
-            return true;
-        }))->willReturn(new Promise(function () { }));
-
-        $this->browser->get('http://example.com/');
-    }
-
     public function testWithHeaderShouldOverwriteExistingHeader()
     {
         $this->browser = $this->browser->withHeader('User-Agent', 'ACMC'); //should be overwritten