Subscriptions: Message HTTP body is missing various fields

[TheJosh]

The JSON body contains some fields such as MessageId, Message, and Type, but other fields are missing.

The PHP SNS SDK requires all of these fields [1] and throws an exception if any are missing:

• Message
• MessageId
• Timestamp
• TopicArn
• Type
• Signature
• SigningCertURL or SigningCertUrl
• SignatureVersion

In my implementation I'm already skipping signing for non-production (my other sns testing tools don't sign the requests) but the fields would still need to exist for the Message object to be constructed, such as via Message::fromRawPostData method.

In my "manually trigger sns handler" testing code code, I'm using the following field values:

Field	Value
MessageId	guid
Signature	EXAMPLE
SignatureVersion	1
SigningCertURL	http://example.com
Timestamp	current unix time, i.e. php gmdate('c')
TopicArn	arn:aws:sns:us-west-2:123456789012:MyTopic but the actual topic would be better
Type	Notification

I think I can see how to patch these other fields in, so I might have a go at it myself.

[1] https://github.com/aws/aws-php-sns-message-validator/blob/master/src/Message.php#L11