LibHTTP: Respect the 'Connection: close' header on keep-alive jobs

If the server responds with this header, we _must_ close the connection,
as the server is allowed to ignore the socket and not respond to
anything past that response.
Fixes some RequestServer spins.

Userland/Libraries/LibCore/Socket.cpp

@@ -208,6 +208,16 @@ void Socket::did_update_fd(int fd)
     }
 }
 
+bool Socket::close()
+{
+    m_connected = false;
+    if (m_notifier)
+        m_notifier->close();
+    if (m_read_notifier)
+        m_read_notifier->close();
+    return IODevice::close();
+}
+
 void Socket::ensure_read_notifier()
 {
     VERIFY(m_connected);