Add guard clause when creating cookie jar

Author: rastating

lib/wpxf/net/http_response.rb

@@ -15,7 +15,7 @@ def parse_typhoeus_response(res)
         self.body = res.body.nil? ? '' : res.body
         self.headers = res.headers
         self.timed_out = res.timed_out? || res.return_code == :couldnt_connect
-        self.cookies = CookieJar.new.parse(res.headers['Set-Cookie'])
+        self.cookies = CookieJar.new.parse(res.headers['Set-Cookie']) if res.headers
       end
 
       # @return [Boolean] a boolean that indicates whether a request timed out.