Remove unnecessary branching. (#143)

Because of how Go's missing-map-keys and
zero value features work together,
the branches for handling MessageGroupId and
MessageDeduplicationId don't achieve anything
that the language doesn't give us for free
(using much less code).

This is a transparent (semantically identical) change.

main.go

@@ -149,21 +149,13 @@ func logAwsError(message string, err error) {
 func convertToEntries(messages []*sqs.Message) []*sqs.SendMessageBatchRequestEntry {
 	result := make([]*sqs.SendMessageBatchRequestEntry, len(messages))
 	for i, message := range messages {
-		requestEntry := &sqs.SendMessageBatchRequestEntry{
-			MessageBody:       message.Body,
-			Id:                message.MessageId,
-			MessageAttributes: message.MessageAttributes,
+		result[i] = &sqs.SendMessageBatchRequestEntry{
+			MessageBody:            message.Body,
+			Id:                     message.MessageId,
+			MessageAttributes:      message.MessageAttributes,
+			MessageGroupId:         message.Attributes[sqs.MessageSystemAttributeNameMessageGroupId],
+			MessageDeduplicationId: message.Attributes[sqs.MessageSystemAttributeNameMessageDeduplicationId],
 		}
-
-		if messageGroupId, ok := message.Attributes[sqs.MessageSystemAttributeNameMessageGroupId]; ok {
-			requestEntry.MessageGroupId = messageGroupId
-		}
-
-		if messageDeduplicationId, ok := message.Attributes[sqs.MessageSystemAttributeNameMessageDeduplicationId]; ok {
-			requestEntry.MessageDeduplicationId = messageDeduplicationId
-		}
-
-		result[i] = requestEntry
 	}
 
 	return result