Cannot erase AWS Lex SessionAttributes

[artempanko]

For erasing aws lex sessionAttributes I need to pass empty map according to aws lex docs (https://docs.aws.amazon.com/lex/latest/dg/context-mgmt.html)
But LexResponse uses json tag with omitempty option and aws lex won't clear sessionAttributes.
Code example: https://play.golang.org/p/Qpca-uHwGQL
I'm proposing to remove omitempty option in LexResponse struct.

[bmoffatt]

Makes sense to me, feel free to open a pull-request!

[artempanko]

Hi, I've created, please review #225