Fix digest auth when multiple headers are sent by the server

Digest authentication would fail if more than one header was sent as
part of the response. Look specifically for the "Digest" word as part of
the www-authenticate header.

This fixes issue jnunemaker#271.

lib/httparty/net_digest_auth.rb

@@ -44,9 +44,9 @@ def authorization_header
     private
 
       def parse(response_header)
-        response_header['www-authenticate'] =~ /^(\w+) (.*)/
+        response_header['www-authenticate'] =~ /Digest (.*)/
         params = {}
-        $2.gsub(/(\w+)="(.*?)"/) { params[$1] = $2 }
+        $1.gsub(/(\w+)="(.*?)"/) { params[$1] = $2 }
         params
       end