Fix memory leak in Client.php

Headers are no longer added every time. They used to be added regardless, which caused the headers array to grow infinitely.

[test] Add headers check for content-type & accept
[cs] Remove whitespace on empty line

Author: ikdekker

test/ClientTest.php

@@ -615,7 +615,7 @@ public function testContentTypeAutomaticallySet()
 
         $this->setServerResponseTo(true);
         $this->assertTrue($this->xmlrpcClient->call('method'));
-        $this->assertSame($expectedContentType, $this->httpClient->getHeader(''));
+        $this->assertSame($expectedContentType, $this->httpClient->getHeader('Content-Type'));
     }
 
     public function testAcceptAutomaticallySet()
@@ -630,7 +630,7 @@ public function testAcceptAutomaticallySet()
 
         $this->setServerResponseTo(true);
         $this->assertTrue($this->xmlrpcClient->call('method'));
-        $this->assertSame($expectedAccept, $this->httpClient->getHeader(''));
+        $this->assertSame($expectedAccept, $this->httpClient->getHeader('Accept'));
     }
 
     /**