negotiate frame_max 4096 with downstream clients

to match the upstream connection frame_max

spec/amqproxy_spec.cr

@@ -250,4 +250,21 @@ describe AMQProxy::Server do
       s.stop_accepting_clients
     end
   end
+
+  it "supports publishing large messages" do
+    s = AMQProxy::Server.new("127.0.0.1", 5672, false)
+    begin
+      spawn { s.listen("127.0.0.1", 5673) }
+      Fiber.yield
+      AMQP::Client.start("amqp://localhost:5673") do |conn|
+        ch = conn.channel
+        q = ch.queue
+        q.publish_confirm Bytes.new(10240)
+        msg = q.get.not_nil!("should not be nil")
+        msg.body_io.bytesize.should eq 10240
+      end
+    ensure
+      s.stop_accepting_clients
+    end
+  end
 end

src/amqproxy/client.cr

@@ -242,7 +242,7 @@ module AMQProxy
       else raise "Unsupported authentication mechanism: #{start_ok.mechanism}"
       end
 
-      tune = AMQ::Protocol::Frame::Connection::Tune.new(frame_max: 131072_u32, channel_max: UInt16::MAX, heartbeat: 0_u16)
+      tune = AMQ::Protocol::Frame::Connection::Tune.new(frame_max: 4096_u32, channel_max: UInt16::MAX, heartbeat: 0_u16)
       tune.to_io(socket, IO::ByteFormat::NetworkEndian)
       socket.flush