[reSIProcate] Asterisk empty `opaque' causes auth exception

[Daniel Pocock]

Asterisk sends the following Proxy-Authorization header:

Proxy-Authorization: Digest username="8000", realm="lvdx.com", 
algorithm=MD5, uri="sip:0208 at lvdx.com", 
nonce="47014...",response="e569d....", opaque=""

Notice that there is no value for `opaque'.  reSIProcate chokes on this. 

The response sent is "400 Invalid auth header"

See the log:

Oct  1 20:15:34 th6 tb2bua: DEBUG | 20071001-201534.899 | tb2bua | RESIP 
| 30728
46048 | BaseException.cxx:17 | BaseException at DataParameter.cxx:54 
DataParamet
er c'tor parsed empty param!
Oct  1 20:15:34 th6 tb2bua: INFO | 20071001-201534.899 | tb2bua | 
RESIP:DUM | 30
72846048 | ServerAuthManager.cxx:342 | Invalid auth header provided 
ParseExcepti
on DataParameter c'tor parsed empty param! @ DataParameter.cxx:54
Oct  1 20:15:34 th6 tb2bua: DEBUG | 20071001-201534.899 | tb2bua | RESIP 
| 30728
46048 | Helper.cxx:372 | Helper::makeResponse(SipReq:  INVITE 44208 at 195.8
.117.23:5060 tid=1daab0f131e7d04109e332ea0bd51bc8 cseq=INVITE contact=8000 at 1
95.8 / 103 from(wire) code=400 reason=Invalid auth header