Batchhandler results in an error when doing bulk updates.

[MitchKuijpers]

Short summary (3-5 sentences) describing the issue.

Assemblies affected

AspNet RESTier 1.1.0

Reproduce steps

Any batch request with more then 1 change.

Expected result

Batch request is properly handled.

Actual result

Batch crashes sometimes, depending on how many changes are sent.

Additional details

RestierBatchChangeSetRequestItem defines a dictionary that is used while firing the individual requests async.