Revert MessageID type change

Author: kdnakt

events/apigw.go

@@ -185,7 +185,7 @@ type APIGatewayWebsocketProxyRequestContext struct {
 	ExtendedRequestID    string                    `json:"extendedRequestId"`
 	IntegrationLatency   string                    `json:"integrationLatency,omitempty"`
 	MessageDirection     string                    `json:"messageDirection"`
-	MessageID            *string                   `json:"messageId,omitempty"`
+	MessageID            string                    `json:"messageId,omitempty"`
 	RequestTime          string                    `json:"requestTime"`
 	RequestTimeEpoch     int64                     `json:"requestTimeEpoch"`
 	RouteKey             string                    `json:"routeKey"`

events/testdata/apigw-websocket-request.json

@@ -97,7 +97,6 @@
     "extendedRequestId": "TWegAcC4EowCHnA=",
     "integrationLatency": "123",
     "messageDirection": "IN",
-    "messageId": "",
     "requestTime": "07/Jan/2019:09:20:57 +0000",
     "requestTimeEpoch": 0,
     "routeKey": "$connect",