Make header value possibly undefined
Make header value possibly undefined

Since the index signature previously claimed that the value
was always `Buffer | string` it means that Typescript would
build even when accessing a header key that isn't guaranteed to
be there, and thus crash at runtime unless a null-check is in place.

Fixes #924.

tulios/kafkajs,tulios/kafkajs,tulios/kafkajs,tulios/kafkajs