correct swap of user_id and username

src/identity.cc

@@ -17,8 +17,8 @@ namespace pEp {
 
         Identity::Identity(string address, string username, string user_id,
                 string fpr, int comm_type, string lang, identity_flags_t flags)
-            : _ident(new_identity(address.c_str(), fpr.c_str(), username.c_str(),
+            : _ident(new_identity(address.c_str(), fpr.c_str(), user_id.c_str(),
-                        user_id.c_str()), &::free_identity)
+                        username.c_str()), &::free_identity)
         {
             if (!_ident)
                 throw bad_alloc();

src/message.cc

@@ -358,6 +358,8 @@ namespace pEp {
 
         static object update(Identity ident)
         {
+            if (ident.address().empty())
+                throw runtime_error("at least address needed");
             update_identity(adapter.session(), ident);
             return object(ident);
         }

test/basic_doctest.py

@@ -1,11 +1,11 @@
 """
 >>> import pEp
->>> me = pEp.Identity("alice.smith@peptest.ch", "23", "Alice Smith")
+>>> me = pEp.Identity("alice.smith@peptest.ch", "Alice Smith", "23")
 >>> me.username
 'Alice Smith'
 >>> print(me)
 Alice Smith <alice.smith@peptest.ch>
->>> you = pEp.Identity("bob.bourne@peptest.ch", "42", "Bob Bourne")
+>>> you = pEp.Identity("bob.bourne@peptest.ch", "Bob Bourne", "42")
 >>> print(you)
 Bob Bourne <bob.bourne@peptest.ch>
 >>> m = pEp.outgoing_message(me)