Set defaults to false for loader definitions

There isn't much value in having defaults set for descriptors

What's more it causes issues when using those descriptors with protobufjs

Descriptors with oneofIndex: 0, cause issues when they aren't part of a oneof

packages/proto-loader/src/index.ts

@@ -148,7 +148,7 @@ const descriptorOptions: Protobuf.IConversionOptions = {
   longs: String,
   enums: String,
   bytes: String,
-  defaults: true,
+  defaults: false,
   oneofs: true,
   json: true,
 };