Fix incorrect deprecation warnings on defaults (#348)

This change ensures that deprecation warnings are only raised when
either a deprecated field is explicitly set or a deprecated message is
initialised.

Resolves: #347

tests/inputs/deprecated/deprecated.json

@@ -1,4 +1,6 @@
 {
-  "v": 10,
+  "message": {
+    "value": "hello"
+  },
   "value": 10
 }

tests/inputs/deprecated/deprecated.proto

@@ -4,8 +4,11 @@ package deprecated;
 
 // Some documentation about the Test message.
 message Test {
-    // Some documentation about the value.
-    option deprecated = true;
-    int32 v = 1 [deprecated=true];
+    Message message = 1 [deprecated=true];
     int32 value = 2;
 }
+
+message Message {
+    option deprecated = true;
+    string value = 1;
+}

tests/inputs/deprecated_field/deprecated_field.json

@@ -1,4 +0,0 @@
-{
-  "v": 10,
-  "value": 10
-}

tests/inputs/deprecated_field/deprecated_field.proto

@@ -1,10 +0,0 @@
-syntax = "proto3";
-
-package deprecated_field;
-
-// Some documentation about the Test message.
-message Test {
-    // Some documentation about the value.
-    int32 v = 1 [deprecated=true];
-    int32 value = 2;
-}

tests/test_deprecated.py

@@ -1,26 +1,42 @@
+import warnings
+
 import pytest
 
-from tests.output_betterproto.deprecated import Test as DeprecatedMessageTest
-from tests.output_betterproto.deprecated_field import Test as DeprecatedFieldTest
+from tests.output_betterproto.deprecated import Message, Test
+
+
+@pytest.fixture
+def message():
+    with warnings.catch_warnings():
+        warnings.filterwarnings("ignore", category=DeprecationWarning)
+        return Message(value="hello")
 
 
 def test_deprecated_message():
-    with pytest.deprecated_call():
-        DeprecatedMessageTest(value=10)
+    with pytest.warns(DeprecationWarning) as record:
+        Message(value="hello")
+
+    assert len(record) == 1
+    assert str(record[0].message) == f"{Message.__name__} is deprecated"
 
 
-def test_deprecated_message_with_deprecated_field():
+def test_message_with_deprecated_field(message):
+    with pytest.warns(DeprecationWarning) as record:
+        Test(message=message, value=10)
+
+    assert len(record) == 1
+    assert str(record[0].message) == f"{Test.__name__}.message is deprecated"
+
+
+def test_message_with_deprecated_field_not_set(message):
     with pytest.warns(None) as record:
-        DeprecatedMessageTest(v=10, value=10)
-    assert len(record) == 2
+        Test(value=10)
+
+    assert not record
+
+
+def test_message_with_deprecated_field_not_set_default(message):
+    with pytest.warns(None) as record:
+        _ = Test(value=10).message
 
-
-def test_deprecated_field_warning():
-    with pytest.deprecated_call():
-        DeprecatedFieldTest(v=10, value=10)
-
-
-def test_deprecated_field_no_warning():
-    with pytest.warns(None) as record:
-        DeprecatedFieldTest(value=10)
     assert not record