IXFR: falback to AXFR master-side fix

When Knot master replied to IXFR query by AXFR-style IXFR, the response was wrong in two ways:
1. Header was cleared: no ID, no QR flag
2. QTYPE was changed to AXFR

Both are wrong. The header must be set properly and the IXFR QTYPE must be preserved even if the response has the AXFR format (that's the way to distinguish it), see [RFC1995, Section 4](http://tools.ietf.org/html/rfc1995#section-4).

I removed these changes to the packet. But I'm not totally sure that there is no problem now, because the packet-processing is too obscure. @mvavrusa should review this definitely.

Tests pending.

See merge request !278