Fix peeking into an already closed socket

Crazily enough, this seems to have been broken ever since a324a7bccf
(Added GSocket for Unix [...], 1999-07-22) which removed the call to
GetPushback(true) from Peek(), making its "peek" parameter unused until
now.

Restore this call in order to return the correct data from the internal
buffer even if the socket has been closed since the last read.

This fixes wxURI tests as the new test HTTP server returns smaller
response which can be typically read entirely at once.

src/common/socket.cpp

@@ -1131,6 +1131,14 @@ wxSocketBase& wxSocketBase::ReadMsg(void* buffer, wxUint32 nbytes)
 
 wxSocketBase& wxSocketBase::Peek(void* buffer, wxUint32 nbytes)
 {
+    // If we're already closed, don't try switching the invalid socket into
+    // non-blocking mode, but still use the already read data, if any.
+    if ( m_impl->m_fd == INVALID_SOCKET )
+    {
+        m_lcount = GetPushback(buffer, nbytes, true);
+        return *this;
+    }
+
     wxSocketReadGuard read(this);
 
     // Peek() should never block