Kinesis Channel Adapters: fix embeddedHeadersMapper and converter logic for batch mode

[artembilan]

We should map records via this.converter in the else block for the embeddedHeadersMapper. We can have an embeddedHeadersMapper and converter, so both of them are used per record. We may have only a converter, so we need to apply it only for records' data. Or we may not have both, so just List<Record>. The case when we don't have a converter, but embeddedHeadersMapper is covered in the prepareMessageForRecord. Right now the problem, that we just ignores an embeddedHeadersMapper result when we also have a converter.