builder pattern for proto construction

SignalServiceKit/src/Protos/ProtoWrappers/SSKEnvelope.swift

@@ -114,11 +114,10 @@ public class SSKEnvelope: NSObject {
     }
 
     private var asProtobuf: SignalServiceProtos_Envelope {
-        var proto = SignalServiceProtos_Envelope()
+        let proto = SignalServiceProtos_Envelope.with { (builder) in
+            builder.source = self.source
 
-        proto.source = self.source
-
-        proto.type = {
+            builder.type = {
                 switch self.type {
                 case .unknown:
                     return .unknown
@@ -133,15 +132,16 @@ public class SSKEnvelope: NSObject {
                 }
             }()
 
-        proto.timestamp = self.timestamp
-        proto.sourceDevice = self.sourceDevice
+            builder.timestamp = self.timestamp
+            builder.sourceDevice = self.sourceDevice
 
             if let relay = self.relay {
-            proto.relay = relay
+                builder.relay = relay
             }
 
             if let content = self.content {
-            proto.content = content
+                builder.content = content
+            }
         }
 
         return proto