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/NetworkJob.cpp

@@ -23,7 +23,7 @@ void NetworkJob::start(NonnullRefPtr<Core::Socket>)
 {
 }
 
-void NetworkJob::shutdown()
+void NetworkJob::shutdown(ShutdownMode)
 {
 }
 
@@ -40,7 +40,7 @@ void NetworkJob::did_finish(NonnullRefPtr<NetworkResponse>&& response)
     dbgln_if(CNETWORKJOB_DEBUG, "{} job did_finish", *this);
     VERIFY(on_finish);
     on_finish(true);
-    shutdown();
+    shutdown(ShutdownMode::DetachFromSocket);
 }
 
 void NetworkJob::did_fail(Error error)
@@ -56,7 +56,7 @@ void NetworkJob::did_fail(Error error)
     dbgln_if(CNETWORKJOB_DEBUG, "{}{{{:p}}} job did_fail! error: {} ({})", class_name(), this, (unsigned)error, to_string(error));
     VERIFY(on_finish);
     on_finish(false);
-    shutdown();
+    shutdown(ShutdownMode::DetachFromSocket);
 }
 
 void NetworkJob::did_progress(Optional<u32> total_size, u32 downloaded)

Userland/Libraries/LibCore/NetworkJob.h

@@ -35,12 +35,16 @@ public:
     NetworkResponse* response() { return m_response.ptr(); }
     const NetworkResponse* response() const { return m_response.ptr(); }
 
+    enum class ShutdownMode {
+        DetachFromSocket,
+        CloseSocket,
+    };
     virtual void start(NonnullRefPtr<Core::Socket>) = 0;
-    virtual void shutdown() = 0;
+    virtual void shutdown(ShutdownMode) = 0;
 
     void cancel()
     {
-        shutdown();
+        shutdown(ShutdownMode::DetachFromSocket);
         m_error = Error::Cancelled;
     }
 

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);

Userland/Libraries/LibCore/Socket.h

@@ -42,6 +42,8 @@ public:
     SocketAddress destination_address() const { return m_destination_address; }
     int destination_port() const { return m_destination_port; }
 
+    virtual bool close() override;
+
     Function<void()> on_connected;
     Function<void()> on_error;
     Function<void()> on_ready_to_read;