Fix batch request handling of sub.sub.subKeys

Limit subKey to not include periods; this is important for sub.sub.subKeys, such as Message Attributes.

E.g. given prefix `SendMessageBatchRequestEntry` and parameters:

    SendMessageBatchRequestEntry.1.MessageAttribute.1.Value.StringValue -> index
    SendMessageBatchRequestEntry.1.MessageBody -> {}
    SendMessageBatchRequestEntry.1.Id -> 0
    Version -> 2012-11-05

we want:

    Map(Id -> 0, MessageAttribute.1.Value.StringValue -> index, MessageBody -> {}, Id -> 0)

I've added a test case to this effect.