SNS MessageAttributes are optional (#208)

motivation: Notice that if you check the contents of the SQS records, some kind of transform happened on the original SNS message. MessageAttributes in the above example, these will not be set, so this is the initial fix.

changes: make MessageAttributes are optional

Author: DwayneCoussement

Diff for: Sources/AWSLambdaEvents/SNS.swift

@@ -49,7 +49,7 @@ public enum SNS {
         public let messageId: String
         public let type: String
         public let topicArn: String
-        public let messageAttributes: [String: Attribute]
+        public let messageAttributes: [String: Attribute]?
         public let signatureVersion: String
 
         @ISO8601WithFractionalSecondsCoding

Diff for: Tests/AWSLambdaEventsTests/SNSTests.swift

@@ -74,9 +74,9 @@ class SNSTests: XCTestCase {
         XCTAssertEqual(record.sns.signingCertURL, "https://sns.eu-central-1.amazonaws.com/SimpleNotificationService-6aad65c2f9911b05cd53efda11f913f9.pem")
         XCTAssertEqual(record.sns.unsubscribeUrl, "https://sns.eu-central-1.amazonaws.com/?Action=Unsubscribe&SubscriptionArn=arn:aws:sns:eu-central-1:079477498937:EventSources-SNSTopic-1NHENSE2MQKF5:6fabdb7f-b27e-456d-8e8a-14679db9e40c")
 
-        XCTAssertEqual(record.sns.messageAttributes.count, 2)
+        XCTAssertEqual(record.sns.messageAttributes?.count, 2)
 
-        XCTAssertEqual(record.sns.messageAttributes["binary"], .binary([UInt8]("base64".utf8)))
-        XCTAssertEqual(record.sns.messageAttributes["string"], .string("abc123"))
+        XCTAssertEqual(record.sns.messageAttributes?["binary"], .binary([UInt8]("base64".utf8)))
+        XCTAssertEqual(record.sns.messageAttributes?["string"], .string("abc123"))
     }
 }