Unicode error causes lookup to throw WhoisLookupError

Test with 68.65.148.250.

The following exception is thrown:

Python 2.6.8 (unknown, Nov 7 2012, 14:47:45)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-52)] on linux2
Type "help", "copyright", "credits" or "license" for more information.

from ipwhois import IPWhois
test=IPWhois('68.65.148.250')
test.lookup()
Traceback (most recent call last):
File "", line 1, in 
File "/usr/lib/python2.6/site-packages/ipwhois/ipwhois.py", line 679, in lookup
response = self.get_whois(results['asn_registry'], retry_count)
File "/usr/lib/python2.6/site-packages/ipwhois/ipwhois.py", line 537, in get_whois
'Whois lookup failed for %r.' % self.address_str
ipwhois.ipwhois.WhoisLookupError: Whois lookup failed for '68.65.148.250'.
Digging deeper, it seems that a unicode conversion error is to blame. Fix attached.