Encode rdata for PTR RRs (Timo Juhani Lindfors)

This imports the change made in commit b33a1b7 to scapy-com repository
by Timo Juhani Lindfors <timo.lindfors@iki.fi>.

This fixes issue #566:

Before:

    >>> DNSRR(str(DNSRR(type="PTR", rdata="secdev.org"))).rdata
    WARNING: DNS RR prematured end (ofs=8550, len=8)
    ''

After:

    >>> DNSRR(str(DNSRR(type="PTR", rdata="secdev.org"))).rdata
    'secdev.org.'

scapy/layers/dns.py

@@ -194,7 +194,7 @@ class RDataField(StrLenField):
         if pkt.type == 1: # A
             if s:
                 s = inet_aton(s)
-        elif pkt.type in [2,3,4,5]: # NS, MD, MF, CNAME
+        elif pkt.type in [2, 3, 4, 5, 12]: # NS, MD, MF, CNAME, PTR
             s = "".join(map(lambda x: chr(len(x))+x, s.split(".")))
             if ord(s[-1]):
                 s += "\x00"