test: unknown fields are preserved (#160)

* test: unknown fields are preserved

Consider the following:

```proto
message Old {
        string name = 1;
}

message New {
        string name = 1;
        string path = 2;
}
```

We can think of `New` as being a minor version release update of `Old`.
If a client using the older version receives a message over the wire
from a server using the newer version, it is desirable that any new,
unknown fields are preserved. These can store context the server
needs, which is important in get/modify/set loops.

proto/message.py

@@ -155,7 +155,6 @@ def __new__(mcls, name, bases, attrs):
 
             # Same thing, but for enums.
             elif field.enum and not isinstance(field.enum, str):
-                field_enum = field.enum
                 field_enum = (
                     field.enum._meta.pb
                     if hasattr(field.enum, "_meta")

tests/test_message.py

@@ -285,6 +285,28 @@ class Octopus_New(proto.Message):
     assert not hasattr(o_old, "length_cm")
 
 
+def test_unknown_field_deserialize_keep_fields():
+    # This is a somewhat common setup: a client uses an older proto definition,
+    # while the server sends the newer definition. The client still needs to be
+    # able to interact with the protos it receives from the server.
+
+    class Octopus_Old(proto.Message):
+        mass_kg = proto.Field(proto.INT32, number=1)
+
+    class Octopus_New(proto.Message):
+        mass_kg = proto.Field(proto.INT32, number=1)
+        length_cm = proto.Field(proto.INT32, number=2)
+
+    o_new = Octopus_New(mass_kg=20, length_cm=100)
+    o_ser = Octopus_New.serialize(o_new)
+
+    o_old = Octopus_Old.deserialize(o_ser)
+    assert not hasattr(o_old, "length_cm")
+
+    o_new = Octopus_New.deserialize(Octopus_Old.serialize(o_old))
+    assert o_new.length_cm == 100
+
+
 def test_unknown_field_from_dict():
     class Squid(proto.Message):
         mass_kg = proto.Field(proto.INT32, number=1)