v1.9.0

A function can now be added to the config object to override the default mechanism used to determine if a call should be batched.
Add a canBatchRequest function to the config object which takes in a url and http method type and return true if the call can be batched.
Add the ability to specify additional header to be added to the batch request and each individual batch request part. This is to improve
compatibly with java servlet <=3.1 where it needs a content disposition header to parse the request.