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;
-}