Increase size of XML decoder internal buffers

Since a transport (and a streamlogger) does not implement io.ByteReader
xml.Decoder wraps it using `bufio.NewReader(transport)` so it can easily read
bytes one at a time. This has the unfortuante effect of resulting in a panic if
we try to parse a stanza that is larger than the default buffer size of 4096
bytes.

To fix this we wrap the transport using `bufio.NewReaderSize()` with a much
larger buffer size.

websocket_transport.go

@@ -1,6 +1,7 @@
 package xmpp
 
 import (
+	"bufio"
 	"context"
 	"encoding/xml"
 	"errors"
@@ -57,7 +58,7 @@ func (t *WebsocketTransport) Connect() (string, error) {
 	t.wsConn = wsConn
 	t.startReader()
 
-	t.decoder = xml.NewDecoder(t)
+	t.decoder = xml.NewDecoder(bufio.NewReaderSize(t, maxPacketSize))
 	t.decoder.CharsetReader = t.Config.CharsetReader
 
 	return t.StartStream()