add instance method Message.set_recv_by

This is necessary in Python applications to correctly parse incoming messages
(currently from MIME) while setting the Recv-By field as required by the Engine.

src/pEp/_pEp/message.cc

@@ -314,6 +314,36 @@ namespace pEp {
             _throw_status(status);
         }
 
+        void Message::set_recv_by(Identity &new_recv_by)
+        {
+            // Validate self and the new Recv-By identity.
+            ::message *msg_c = _msg.get();
+            if (msg_c->dir != ::PEP_dir_incoming) {
+                throw invalid_argument("message not incoming");
+            }
+            if (new_recv_by.address().empty() || new_recv_by.user_id().empty()) {
+                throw invalid_argument("identity incomplete");
+            }
+            if (new_recv_by.user_id() != PEP_OWN_USERID) {
+                throw invalid_argument("identity non-own");
+            }
+
+            // Make a a copy of the given identity.
+            ::pEp_identity *new_recv_by_c = new_recv_by;
+            ::pEp_identity *new_recv_by_copy_c = ::identity_dup(new_recv_by_c);
+            if (new_recv_by_copy_c == NULL) {
+                throw std::bad_alloc();
+            }
+            // If we have not failed yet this will succeed.
+
+            // Destroy the old Recv-By.
+            ::free_identity(msg_c->recv_by);
+            msg_c->recv_by = NULL;
+
+            // Use the copy as the new Recv-By.
+            msg_c->recv_by = new_recv_by_copy_c;
+        }
+
         Message Message::encrypt()
         {
             boost::python::list extra;

src/pEp/_pEp/message.hh

@@ -303,6 +303,8 @@ namespace pEp {
             void remove_opt_field(string name);
             void set_opt_field(string name, string value);
 
+            void set_recv_by(Identity &new_recv_by);
+
             Message encrypt();
 
             Message _encrypt(boost::python::list extra, int enc_format = 4, int flags = 0);

src/pEp/_pEp/pEpmodule.cc

@@ -551,6 +551,13 @@ namespace pEp {
                                           "   msg            a message\n"
                                           "   name           the field name as a string\n"
                                           "   value          the new field value as a string\n")
+                                     .def("set_recv_by", &Message::set_recv_by,
+                                          "msg.set_recv_by(identity)\n"
+                                          "\n"
+                                          "Alters the given incoming message setting or replacing the Recv-By\n"
+                                          "field with the given own identity.\n"
+                                          "\n"
+                                          "   identity       an own identity, on which myself has been called already\n")
                                      .def("encrypt", (Message(Message::*)()) & Message::encrypt)
                                      .def(
                                          "encrypt",